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According to the failure model (Patterson & Capaldi, 1990), peer rejection is the intermediary link between problem behaviors and internalizing symptoms. The present study tested the model with 464 monozygotic and same-sex dizygotic twin pairs (234 female, 230 male dyads). Teacher-reported reactive aggression and internalizing symptoms, and peer-reported peer rejection were collected at ages 6, 7, and 10 (from 2001 to 2008). Support for the failure model emerged in conventional non-genetically controlled analyses, but not twin-difference score analyses (which remove shared environmental and genetic contributions). Univariate biometric models attributed minimal variance in failure model variables to shared environmental factors, suggesting that genetic factors play an important unacknowledged role in developmental pathways historically ascribed to nonshared experiences in the failure model., (© 2023 The Authors. Child Development © 2023 Society for Research in Child Development.)

Background: Population-based and family studies showed that impulsive-aggression predicts suicidality; however, the underlying etiological nature of this association is poorly understood. The objective was to determine the contribution of genes and environment to the association between childhood impulsive-aggression and serious suicidal ideation/attempt in young adulthood., Methods: N = 862 twins (435 families) from the Quebec Newborn Twin Study were followed up from birth to 20 years. Repeated measures of teacher-assessed impulsive-aggression were modeled using a genetically informed latent growth model including intercept and slope parameters reflecting individual differences in the baseline level (age 6 years) and in the change (increase/decrease) of impulsive-aggression during childhood (6 to 12 years), respectively. Lifetime suicidality (serious suicidal ideation/attempt) was self-reported at 20 years. Associations of impulsive-aggression intercept and slope with suicidality were decomposed into additive genetic (A) and unique environmental (E) components., Results: Additive genetic factors accounted for an important part of individual differences in impulsive-aggression intercept (A = 90%, E = 10%) and slope (A = 65%, E = 35%). Genetic (50%) and unique environmental (50%) factors equally contributed to suicidality. We found that 38% of the genetic factors accounting for suicidality were shared with those underlying impulsive-aggression slope, whereas 40% of the environmental factors accounting for suicidality were shared with those associated with impulsive-aggression intercept. The genetic correlation between impulsive-aggression slope and suicidality was 0.60, p = .027., Conclusions: Genetic and unique environmental factors underlying suicidality significantly overlap with those underlying childhood impulsive-aggression. Future studies should identify putative genetic and environmental factors to inform prevention., (© 2019 Association for Child and Adolescent Mental Health.)

Background: Child-care services during early childhood provide opportunities for social interactions that may facilitate children's learning of acceptable social behaviors. Furthermore, they may reduce exposure to family adversity for some children. The aim of this study was to determine whether intensity of exposure to child-care services prior to age 5 years has a beneficial effect on disruptive behavior problems during adolescence, and whether the effect is more pronounced for children from low socioeconomic families., Methods: N = 1,588 participants from the Québec Longitudinal Study of Child Development were assessed 14 times from 5 months to 17 years. Intensity of child-care exposure was measured from 5 months to 5 years of age. Main outcomes were self-reported physical aggression and opposition from age 12 to 17 years. Family socioeconomic status (SES) was measured at 5 months. Factors explaining differences in child-care use were controlled using propensity score weights (PSW)., Results: Children exposed to moderate-intensity child-care services (part-time child-care services before 1½ years and full time afterward) reported lower levels of physical aggression (d = -.11, p = .056) and opposition (d = -.14, p = .029) during adolescence compared to children exposed to low-intensity child-care services. A significant child care by SES interaction (p = .017) for physical aggression indicated that the moderate-intensity child-care effect was specific to children from low SES families (d = -.36, p = .002). No interaction with socioeconomic status was found for opposition., Conclusions: Moderate-intensity child-care services from infancy to school entry may prevent disruptive behavior during adolescence, especially for disadvantaged children., (© 2019 Association for Child and Adolescent Mental Health.)

Importance: This study used multitrajectory modeling to identify distinct trajectories of physical aggression from ages 1.5 to 13 years for boys and girls., Objectives: To trace the development of boys' and girls' physical aggression problems from infancy to adolescence using mother ratings, teacher ratings, and self-ratings and to identify early family predictors of children on the high physical aggression trajectories., Design, Setting, and Participants: This cohort study used data from the Quebec Longitudinal Study of Child Development (QLSCD), a study of a representative, population-based sample of 2223 infants born in 1997 and 1998 in the Canadian province of Quebec. The dates of analysis were January 2017 to January 2018., Main Outcomes and Measures: Trained research assistants conducted 7 interviews (at child ages 1.5, 2.5, 3.5, 4.5, 5, 6, and 8 years) with the person most knowledgeable about the child (mothers in 99.6% [2214 of 2223] of cases). Teachers assessed the child's behavior at ages 6, 7, 8, 10, 12, and 13 years. Self-reports of behavior problems were obtained from the child at ages 10, 12, and 13 years., Results: The sample included 2223 participants, 51.2% of whom were boys and 91.2% of whom were of white race/ethnicity. The mean response rate for mother ratings of physical aggression during the first 8 years of life was 80.9% (range, 65.1%-91.7%). For teacher ratings of physical aggression from ages 6 to 13 years, the mean response rate was 45.7% (range, 35.4%-56.9%), while the mean response rate of physical aggression assessment from self-ratings between ages 10 and 13 years was 57.9% (range, 55.2%-60.5%). Attrition was higher among families with low socioeconomic status and single-parent families, as well as among young mothers and mothers who were not fluent in French or English. A statistical analysis to examine the consequences of attrition was included. For boys and girls, the frequency of physical aggressions increased from age 1.5 years (2039 [91.7%]) to age 3.5 years (1941 [87.3%]) and then substantially decreased until age 13 years (1228 [55.2%]). Three distinct developmental trajectories of physical aggression were observed for girls and 5 for boys. Most family characteristics measured at 5 months after the child's birth were associated with a high physical aggression trajectory for boys and girls., Conclusions and Relevance: Family characteristics at 5 months after the child's birth could be used to target preschool interventions aimed at preventing the development of boys' and girls' chronic physical aggression problems.

Background: Few studies are grounded in a developmental framework to study proactive and reactive aggression. Furthermore, although distinctive correlates, predictors and outcomes have been highlighted, proactive and reactive aggression are substantially correlated. To our knowledge, no empirical study has examined the communality of genetic and environmental underpinning of the development of both subtypes of aggression. The current study investigated the communality and specificity of genetic-environmental factors related to heterogeneity in proactive and reactive aggression's development throughout childhood., Methods: Participants were 223 monozygotic and 332 dizygotic pairs. Teacher reports of aggression were obtained at 6, 7, 9, 10 and 12 years of age. Joint development of both phenotypes were analyzed through a multivariate latent growth curve model. Set point, differentiation, and genetic maturation/environmental modulation hypotheses were tested using a biometric decomposition of intercepts and slopes., Results: Common genetic factors accounted for 64% of the total variation of proactive and reactive aggression's intercepts. Two other sets of uncorrelated genetic factors accounted for reactive aggression's intercept (17%) on the one hand, and for proactive (43%) and reactive (13%) aggression's slopes on the other. Common shared environmental factors were associated with proactive aggression's intercept (21%) and slope (26%) and uncorrelated shared environmental factors were also associated with reactive aggression's slope (14%). Common nonshared environmental factors explained most of the remaining variability of proactive and reactive aggression slopes., Conclusions: A genetic differentiation hypothesis common to both phenotypes was supported by common genetic factors associated with the developmental heterogeneity of proactive and reactive aggression in childhood. A genetic maturation hypothesis common to both phenotypes, albeit stronger for proactive aggression, was supported by common genetic factors associated with proactive and reactive aggression slopes. A shared environment set point hypothesis for proactive aggression was supported by shared environmental factors associated with proactive aggression baseline and slope. Although there are many common features to proactive and reactive aggression, the current research underscores the advantages of differentiating them when studying aggression.

Using a sample of 767 children (403 girls, 364 boys), this study aimed to (a) identify groups with distinct trajectories of peer victimization over a 6-year period from primary school through the transition to secondary school, and (b) examine the associated personal (i.e., aggression or internalizing problems) and familial (family status, socioeconomic status, the parent-child relationship) predictors. Peer victimization was assessed via self-reports from Grades 4 through 9 (ages 10 through 15 years), aggression and internalizing problems were assessed in Grade 4 via peer nominations, and the parent-child relationship was assessed in Grade 7 (i.e., right after the transition to secondary school) via parent-reports. Growth Mixture modeling revealed 1 group (62%) who experienced little victimization in primary school and even less in secondary school, another group (31%) who was victimized in primary but not or much less in secondary school, and a third group (7%) who was chronically victimized in both school contexts. Boys were more likely than girls to follow any elevated victimization trajectory. Chronic victimization across primary and secondary school was predicted by nonintact family status and a combination of both internalizing problems and aggression compared with nonvictimized youth. In contrast, transitory victimization during primary but not in secondary school was predicted by aggression, but not internalizing problems. Support as well as conflict in the parent-child relationship also showed significant, albeit distinct associations with the different peer victimization trajectories. (PsycINFO Database Record, ((c) 2016 APA, all rights reserved).)

Comorbidity is frequent among disruptive behaviors (DB) and leads to mental health problems during adolescence and adulthood. However, the early developmental origins of this comorbidity have so far received little attention. This study investigated the developmental comorbidity of three DB categories during early childhood: hyperactivity-impulsivity, non-compliance, and physical aggression. Joint developmental trajectories of DB were identified based on annual mother interviews from age 1½ to 5 years, in a population-representative birth-cohort (N = 2045). A significant proportion of children (13 % to 21 %, depending on the type of DB) consistently displayed high levels of hyperactivity-impulsivity, non-compliance, or physical aggression from age 1½ to 5 years. Developmental comorbidity was frequent, especially for boys: 10 % of boys and 3.7 % of girls were on a stable trajectory with high levels of symptoms for the three categories of DB. Significant associations were observed between preschool joint-trajectories of DB and indicators of DB and school adjustment assessed by teachers in first grade. Preschoolers who maintained high levels of hyperactivity-impulsivity, non-compliance, and physical aggression, displayed the highest number of DB symptoms in first grade for all categories according to their teacher. They were also among the most disadvantaged of their class for school adjustment indicators. Thus, DB manifestations and developmental comorbidity of DB are highly prevalent in infancy. Early childhood appears to be a critical period to prevent persistent and comorbid DB that leads to impairment at the very beginning of school attendance and to long-term serious health and social adjustment problems.

The present study examined sibling influence over reactive and proactive aggression in a sample of 452 same-sex twins (113 male dyads, 113 female dyads). Between and within siblings influence processes were examined as a function of relative levels of parental coercion and hostility to test the hypothesis that aggression contagion between twins occurs only among dyads who experience parental coerciveness. Teacher reports of reactive and proactive aggression were collected for each twin in kindergarten (M = 6.04 years; SD = 0.27) and in first grade (M = 7.08 years; SD = 0.27). Families were divided into relatively low, average, and relatively high parental coercion-hostility groups on the basis of maternal reports collected when the children were 5 years old. In families with relatively high levels of parental coercion-hostility, there was evidence of between-sibling influence, such that one twin's reactive aggression at age 6 predicted increases in the other twin's reactive aggression from ages 6 to 7, and one twin's proactive aggression at age 6 predicted increases in the other twin's proactive aggression from ages 6 to 7. There was also evidence of within-sibling influence such that a child's level of reactive aggression at age 6 predicted increases in the same child's proactive aggression at age 7, regardless of parental coercion-hostility. The findings provide new information about the etiology of reactive and proactive aggression and individual differences in their developmental interplay., (© 2015 Wiley Periodicals, Inc.)

Numerous studies have shown that aggressive and non-aggressive antisocial behaviors are important precursors of later adjustment problems. There is also strong empirical evidence that both types of antisocial behavior are partially influenced by genetic factors. However, despite its important theoretical and practical implications, no study has examined the question whether environmental factors differentially moderate the expression of genetic influences on the two types of antisocial behavior. Using a genetically informed design based on 266 monozygotic and dizygotic twin pairs, this study examined whether the expression of genetic risk for aggressive and non-aggressive antisocial behavior varies depending on the peer group's injunctive norms (i.e., the degree of acceptability) of each type of antisocial behavior. Self-reported aggressive and non-aggressive antisocial behavior and classroom-based sociometric nominations were collected when participants were 10 years old. Multivariate genetic analyses revealed some common genetic factors influencing both types of antisocial behavior (i.e., general antisocial behavior) as well as genetic influences specific to non-aggressive antisocial behavior. However, genetic influences on general antisocial behavior, as well as specific genetic influences on non-aggressive antisocial behavior, vary depending on the injunctive classroom norms regarding these behaviors. These findings speak to the power of peer group norms in shaping aggressive and non-aggressive antisocial behavior. They also contribute further to understanding the distinctive development of both types of antisocial behavior. Finally, they may have important implications for prevention purposes.

Using a genetically informed design based on 197 Monozygotic and Dizygotic twin pairs assessed in grade 4, this study examined 1) whether, in line with a gene-environment correlation (rGE), a genetic disposition for physical aggression or relational aggression puts children at risk of being victimized by their classmates, and 2) whether this rGE is moderated by classroom injunctive norm salience in regard to physical or relational aggression. Physical aggression and relational aggression, as well as injunctive classroom norm salience in regard to these behaviors, were measured via peer nominations. Peer victimization was measured via self-reports. Multi-Level Mixed modeling revealed that children with a genetic disposition for either aggressive behavior are at higher risk of being victimized by their peers only when classroom norms are unfavourable toward such behaviors. However, when classroom injunctive norms favor aggressive behaviors, a genetic disposition for physical or relational aggression may actually protect children against peer victimization. These results lend further support to the notion that bullying interventions must include the larger peer context instead of a sole focus on victims and bullies.

Background: Does poor language ability in early childhood increase the likelihood of physical aggression or is language ability delayed by frequent physical aggression? This study examined the longitudinal associations between physical aggression and language ability from toddlerhood to early childhood in a population sample while controlling for parenting behaviours, non-verbal intellectual functioning, and children's sex., Methods: Children enrolled in the Quebec Longitudinal Study of Child Development (QLSCD) (N = 2, 057) were assessed longitudinally from 17 to 72 months via parent reports and standardized assessments., Results: The cross-lagged models revealed modest reciprocal associations between physical aggression and language performance from 17 to 41 months but not thereafter., Conclusions: Significant associations between physical aggression and poor language ability are minimal and limited to the period when physical aggression and language performance are both substantially increasing. During that period parenting behaviours may play an important role in supporting language ability while reducing the frequency of physical aggression. Further studies are needed that utilize multiple assessments of physical aggression, assess multiple domains of language abilities, and that examine the potential mediating role of parenting behaviours between 12 and 48 months.

Several authors consider high and frequent conflicts between friends during childhood as a serious risk for subsequent conduct problems such as generalized physical aggression toward others (e.g., Kupersmidt, Burchinal, & Patterson, 1995; Sebanc, 2003). Although it seems logical to assume that friendship conflict could have some negative consequences on children's behaviors, some scholars have suggested that a certain amount of conflict between friends may actually promote social adjustment (e.g., Laursen & Pursell, 2009). The aim of this study was to investigate the role of friendship conflict in regard to the development of generalized physical aggression toward others in the early school years (i.e., from kindergarten to Grade 1), as well as the moderating role of relational (i.e., shared positive affect and dyadic conflict resolution skills) and personal (i.e., children's sex and genetic liability for aggression) characteristics in this context. The sample included 745 twins assessed through teacher, peer, child, and friend ratings in kindergarten and Grade 1. Friendship conflict in kindergarten was linearly related to an increase in boys' but not girls' generalized physical aggression. However, shared positive affect and conflict resolution skills mitigated the prospective associations between friendship conflict and generalized physical aggression. These results were independent of children's sex, genetic risk for physical aggression, and initial levels of generalized physical aggression in kindergarten. Fostering a positive relationship between friends at school entry may buffer against the risk associated with experiencing friendship conflict., (PsycINFO Database Record (c) 2014 APA, all rights reserved.)

Using a genetically informed design, this study examined the additive and interactive effects of genetic risk, personal peer victimization experiences, and peer victimization experienced by others on children's aggression and depression symptoms. Of major interest was whether these effects varied depending on whether or not the victimized others were children's close friends. The sample comprised 197 monozygotic and same-sex dizygotic twin pairs reared together (95 female pairs) assessed in Grade 4. Each twin's victimization experiences and victimization experienced by his or her friends and other classmates were measured using individuals' reports about their own levels of peer victimization. Aggression was assessed using peer nominations, and depression was measured using self-reports. Indicative of a possible social-learning mechanism or the emotional contagion of anger, multilevel regressions showed that personal victimization experiences were related to especially high levels of aggression when close friends where also highly victimized, albeit only in boys. Moreover, in line with social comparison theory, the effect of frequent personal victimization experiences on depressive feelings was much weaker when close friends were also highly victimized than when close friends were not or were only rarely victimized. Finally, a high level of peer victimization experienced by other classmates was related to a lower level of aggression in girls and boys, possibly because of a heightened sense of threat in classrooms where many suffer attacks from bullies. All of these results were independent of children's genetic risk for aggression or depression. Theoretical and practical implications are discussed., ((c) 2013 APA, all rights reserved.)

Aggressive behavior in middle childhood is at least partly explained by genetic factors. Nevertheless, estimations of simple effects ignore possible gene-environment interactions (G × E) or gene-environment correlations (rGE) in the etiology of aggression. The present study aimed to simultaneously test for G × E and rGE processes between aggression, on the one hand, and peer victimization and the teacher-child relationship in school, on the other hand. The sample comprised 124 MZ pairs and 93 DZ pairs assessed in Grade 1 (mean age = 84.7 months). Consistent with rGE, children with a presumed genetic disposition for aggression were at an increased risk of peer victimization, whereas in line with G × E, a positive relationship with the teacher mitigated the genetically mediated expression of aggression., (© 2011 The Authors. Child Development © 2011 Society for Research in Child Development, Inc.)

This study used the monozygotic (MZ) twin difference method to examine whether differences in friends' aggression increased the differences in MZ twins' aggression and depressive symptoms from kindergarten to Grade 1 and whether perceived victimization by the friend played a mediating role in this context. Participants were 223 MZ twin pairs. Results showed that differences in kindergarten friends' aggression significantly predicted an increased difference in MZ twins' aggression from kindergarten (mean age = 6.7 years) to Grade 1 (mean age = 7.5 years) for both boys and girls. Differences in perceived victimization by the friend mediated this association, albeit only in boys. Differences in perceived victimization by the friend also predicted an increase in MZ twins' differences in depressive symptoms. These results support the importance of friendship experiences during early childhood., (© 2011 The Authors. Child Development © 2011 Society for Research in Child Development, Inc.)

This study investigated the relation between theory of mind and reactive and proactive aggression, respectively, as well as the moderating role of peer victimization in this context. The 574 participants were drawn from a longitudinal study of twins. Theory of mind was assessed before school entry, when participants were 5 years old. Reactive and proactive aggression as well as peer victimization were assessed a year later in kindergarten. Results from multilevel regression analyses revealed that low theory of mind was related to a high level of reactive aggression, but only in children who experienced average to high levels of peer victimization. In contrast, a high theory of mind was related to a high level of proactive aggression. Again, this relation was especially pronounced in children who experienced high levels of peer victimization. These findings challenge the social skills deficit view of aggression and provide support for a multidimensional perspective of aggressive behavior.

Although peer victimization places children at serious risk for aggressive behavior, not all victimized children are aggressive. The diathesis-stress hypothesis of disease proposes that an environmental stressor such as peer victimization should to lead to maladjustment mostly in those individuals with preexisting genetic vulnerabilities. Accordingly, this study examined whether the link between peer victimization and child aggression is moderated by children's genetic risk for such behavior. Using a sample of 506 6-year-old twins, peer victimization was assessed through peer nominations and aggressive behavior was assessed through peer and teacher reports. Children's genetic risk for aggression was estimated as a function of their co-twin's aggression and the pair's zygosity. Genetic modeling showed that peer victimization is an environmentally driven variable that is unrelated to children's genetic disposition. Results also provided support for the notion of a gene-environment interaction between peer victimization and child's genetic risk for aggressive behavior, albeit only in girls. For boys, peer victimization was related to aggression regardless of the child's genetic risk for such behavior. Different socialization experiences in girls' compared to boys' peer groups may explain the different pattern of results for girls and boys.

This study investigated joint effects of maternal prenatal smoking and parental history of antisocial behavior on physical aggression between ages 17 and 42 months in a population sample of children born in Québec (N = 1,745). An analysis of variance (ANOVA) showed significant main effects of maternal prenatal smoking and a significant interaction between maternal prenatal smoking and mother's history of antisocial behavior in the prediction of children's probability to display high and rising physical aggression. The interaction indicated that the effects of heavy smoking during pregnancy (> or =10 cigarettes/day) were greater when the mother also had a serious history of antisocial behavior. The effects remained significant after the introduction of control variables (e.g., hostile-reactive parenting, family functioning, parental separation/divorce, family income, and maternal education). Another significant interaction not accounted for by control variables was observed for maternal prenatal smoking and family income, indicating more serious effects of maternal prenatal smoking under relatively low-income, conditions. Both interactions indicate critical adversities that, in combination with maternal prenatal smoking, have supra-additive effects on (the development of) physical aggression during early childhood. These findings may have implications for the selection of intervention targets and strategies.

Based on a sample of 406 seven-year-old twins, this study examined whether exposure to friends' social or physical aggression, respectively, moderates the effect of heritability on children's own social and physical aggression. Univariate analyses showed that children's own social and physical aggression were significantly explained by genetic factors, whereas friends' social and physical aggression represented "true" environmental factors that were unrelated to children's genetic dispositions. Multivariate analyses further suggested a possible gene-environment interaction in the link between friends' and children's physical aggression but not in the link between friends' and children's social aggression. Instead, friends' social aggression was directly related to children's social aggression, in addition to genetic effects on this behavior. Theoretical and practical implications of these results are discussed.

Context: Physical violence is an important health problem, and low maternal education is a significant risk for the development of chronic physical aggression (PA). We hypothesized that nonmaternal care (NMC) services could prevent the development of childhood PA problems, depending on the age at which the services are initiated. Method Children who followed a trajectory of atypically frequent PA between 17 and 60 months of age among a population sample of 1691 Canadian families were identified. Maternal education and NMC were considered in predicting group membership while controlling for confounding family characteristics., Results: Children of mothers with low education levels (ie, no high school diploma) were less likely to receive NMC. Those who did receive such care had significantly lower risk of a high PA trajectory. Results from logistic regressions indicated that NMC reduced the risk of high PA, especially when initiated before age 9 months (odds ratio, 0.20; 95% confidence interval, 0.05-0.90). Children of mothers who graduated from high school were less at risk of PA problems, and NMC had no additional protective effect., Conclusions: Nonmaternal care services to children of mothers with low levels of education could substantially reduce their risk of chronic PA, especially if provided soon after birth. Because children most likely to benefit from NMC services are less likely to receive them, special measures encouraging the use of NMC services among high-risk families are needed.

Objective: To examine whether kindergarten children's genetic liability to physically aggress moderates the contribution of friends' aggression to their aggressive behaviors., Method: Teacher and peer reports of aggression were available for 359 6-year-old twin pairs (145 MZ, 212 DZ) as well as teacher and peer reports of aggression of the two best friends of each twin. Children's genetic risk for aggression was based on their cotwin's aggression status and the pair's zygosity., Results: Children's aggression was highly heritable. Unique environment accounted for most of the variance in friends' aggression, although there was also a small genetic contribution (15%). Both genetic liability to aggression and having aggressive friends predicted twins' aggression. However, the contribution of aggressive friends to children's aggression was strongest among genetically vulnerable children. This result was similar for boys and girls, despite sex differences in both aggression and the level of aggression of friends., Conclusions: Affiliation with aggressive friends at school entry is a significant environmental risk factor for aggression, especially for children genetically at risk for aggressive behaviors. Developmental models of aggression need to take into account both genetic liability and environmental factors in multiple settings, such as the peer context, to more precisely describe and understand the various developmental pathways to aggression. The implications for early prevention programs are discussed.

This study examined (a) the predictive link between peer victimization and children's reactive and proactive aggression, and (b) the potential moderating effect of reciprocal friends' reactive and proactive aggression in this context. The study also examined whether these potential moderating effects of friends' characteristics were stronger with respect to more recent friends compared to previous friends. Based on a convenience sample of 658 twin children (326 boys and 332 girls) assessed in kindergarten and first grade, the results showed that peer victimization uniquely predicted an increase in children's teacher-rated reactive aggression, but not teacher-rated proactive aggression. The relation of peer victimization to increased reactive aggression was, however, moderated by recent not previous reciprocal friends' similarly aggressive characteristics. These findings, however, tended to be mostly true for boys, but not for girls. The findings are discussed in terms of their theoretical and practical implications for victimized children's risk of displaying reactive and proactive aggressive behaviors.

This study investigated associations between maternal prenatal smoking and physical aggression (PA), hyperactivity-impulsivity (HI) and co-occurring PA and HI between ages 17 and 42 months in a population sample of children born in Québec (Canada) in 1997/1998 (N=1745). Trajectory model estimation showed three distinct developmental patterns for PA and four for HI. Multinomial regression analyses showed that prenatal smoking significantly predicted children's likelihood to follow different PA trajectories beyond the effects of other perinatal factors, parental psychopathology, family functioning and parenting, and socio-economic factors. However, prenatal smoking was not a significant predictor of HI in a model with the same control variables. Further multinomial regression analyses showed that, together with gender, presence of siblings and maternal hostile reactive parenting, prenatal smoking independently predicted co-occurring high PA and high HI compared to low levels of both behaviors, to high PA alone, and to high HI alone. These results show that maternal prenatal smoking predicts multiple behavior regulation problems in early childhood.

There has been much controversy over the past decades on the origins of gender differences in children's aggressive behavior. A widely held view is that gender differences emerge sometime after 2 years of age and increase in magnitude thereafter because of gender-differentiated socialization practices. The objective of this study was to test for (a) gender differences in the prevalence of physical aggression in the general population of 17-month-old children and (b) change in the magnitude of these differences between 17 and 29 months of age. Contrary to the differential socialization hypothesis, the results showed substantial gender differences in the prevalence of physical aggression at 17 months of age, with 5% of boys but only 1% of girls manifesting physically aggressive behaviors on a frequent basis. The results suggest that there is no change in the magnitude of these differences between 17 and 29 months of age., (Copyright 2006 APA, all rights reserved.)

This study compared the contribution of genes and environment to teacher-rated reactive and proactive aggression in 6-year-old twin pairs (172 pairs: 55 monozygotic girls, 48 monozygotic boys, 33 dizygotic girls, 36 dizygotic boys). Genetic effects accounted for 39% of the variance of reactive aggression and for 41% of the variance of proactive aggression. The remainder of the variance was explained by unique environmental effects. Genetic as well as unique environmental effects were significantly correlated across reactive and proactive aggression (genetic correlation = .87, environmental correlation = .34), but this overlap was largely due to a common underlying form of aggression (i.e., teacher-rated physical aggression). Once common etiological factors due to physical aggression were accounted for, reactive and proactive aggression shared no other genes and only a few environmental influences, although additional specific genetic and environmental effects were observed for both reactive and proactive aggression. These specific effects indicate that both reactive and proactive aggression may be influenced mostly by socialization experiences that are specific to each type of aggression and only to a very small degree by specific genes.

The goal of this study was to examine the links between difficult temperament (i.e., negative emotionality) and harsh parental discipline during toddlerhood, and reactive and proactive aggression in kindergarten. These links were assessed on a longitudinal population-based study of 1516 boys and girls followed longitudinally from the age of 17 months through the age of 72 months. Two possible models were tested to examine the interplay between negative emotionality and harsh parenting in predicting later reactive aggression compared to proactive aggression. The first was an additive model where both aspects make unique contributions in predicting later reactive aggression. The second model was an interactive model where harsh parenting exacerbates the link between negative emotionality and reactive aggression. Results showed a specific contribution of negative emotionality to reactive aggression. The results relative to harsh parenting are more mixed but nonetheless in line with developmental models stressing different pathways to reactive and proactive aggression.

Using a genetic design of 234 six-year-old twins, this study examined (a) the contribution of genes and environment to social versus physical aggression, and (b) whether the correlation between social and physical aggression can be explained by similar genetic or environmental factors or by a directional link between the phenotypes. For social aggression, substantial (shared and unique) environmental effects but only weak genetic effects were found. For physical aggression, significant effects of genes and unique environment were found. Bivariate modeling suggests that social and physical aggression share most of their underlying genes but only very few overlapping environmental factors. The correlation between the two phenotypes can also be explained by a directional effect from physical to social aggression.

Objectives: Physical aggression in children is a major public health problem. Not only is childhood physical aggression a precursor of the physical and mental health problems that will be visited on victims, but also aggressive children themselves are at higher risk of alcohol and drug abuse, accidents, violent crimes, depression, suicide attempts, spouse abuse, and neglectful and abusive parenting. Furthermore, violence commonly results in serious injuries to the perpetrators themselves. Although it is unusual for young children to harm seriously the targets of their physical aggression, studies of physical aggression during infancy indicate that by 17 months of age, the large majority of children are physically aggressive toward siblings, peers, and adults. This study aimed, first, to identify the trajectories of physical aggression during early childhood and, second, to identify antecedents of high levels of physical aggression early in life. Such antecedents could help to understand better the developmental origins of violence later in life and to identify targets for preventive interventions., Methods: A random population sample of 572 families with a 5-month-old newborn was recruited. Assessments of physical aggression frequency were obtained from mothers at 17, 30, and 42 months after birth. Using a semiparametric, mixture model, distinct clusters of physical aggression trajectories were identified. Multivariate logit regression analysis was then used to identify which family and child characteristics, before 5 months of age, predict individuals on a high-level physical aggression trajectory from 17 to 42 months after birth., Results: Three trajectories of physical aggression were identified. The first was composed of children who displayed little or no physical aggression. These individuals were estimated to account for approximately 28% of the sample. The largest group, estimated at approximately 58% of the sample, followed a rising trajectory of modest aggression. Finally, a group, estimated to comprise approximately 14% of the sample, followed a rising trajectory of high physical aggression. Best predictors before or at birth of the high physical aggression trajectory group, controlling for the levels of the other risk factors, were having young siblings (odds ratio [OR]: 4.00; confidence interval [CI]: 2.2-7.4), mothers with high levels of antisocial behavior before the end of high school (OR: 3.1; CI: 1.1-8.6), mothers who started having children early (OR: 3.1; CI: 1.4-6.8), families with low income (OR: 2.6; CI: 1.3-5.2), and mothers who smoked during pregnancy (OR: 2.2; CI: 1.1-4.1). Best predictors at 5 months of age were mothers' coercive parenting behavior (OR: 2.3; CI: 1.1-4.7) and family dysfunction (OR: 2.2; CI: 1.2-4.1). The OR for a high-aggression trajectory was 10.9 for children whose mother reported both high levels of antisocial behavior and early childbearing., Conclusions: Most children have initiated the use of physical aggression during infancy, and most will learn to use alternatives in the following years before they enter primary school. Humans seem to learn to regulate the use of physical aggression during the preschool years. Those who do not, seem to be at highest risk of serious violent behavior during adolescence and adulthood. Results from the present study indicate that children who are at highest risk of not learning to regulate physical aggression in early childhood have mothers with a history of antisocial behavior during their school years, mothers who start childbearing early and who smoke during pregnancy, and parents who have low income and have serious problems living together. All of these variables are relatively easy to measure during pregnancy. Preventive interventions should target families with high-risk profiles on these variables. Experiments with such programs have shown long-term impacts on child abuse and child antisocial behavior. However, these impacts were not observed in families with physical violence. The problem may be that the prevention programs that were provided did not specifically target the parents' control over their physical aggression and their skills in teaching their infant not to be physically aggressive. Most intervention programs to prevent youth physical aggression have targeted school-age children. If children normally learn not to be physically aggressive during the preschool years, then one would expect that interventions that target infants who are at high risk of chronic physical aggression would have more of an impact than interventions 5 to 10 years later, when physical aggression has become a way of life.

The purpose of the present study was to examine whether indirect aggression was distinct from physical aggression across developmental periods. Participants were 3,089 Canadian children aged 4 to 7 years (Time 1), 6 to 9 years (Time 2), and 8 to 11 years (Time 3). Confirmatory factor analysis using an accelerated longitudinal design confirmed a 2-factor model that was stable across cohorts, time, and sex. The longitudinal predictive links between indirect and physical aggression were also examined in a path analysis. Findings did not support the notion that maturation is associated with changes in the ways children aggress but rather suggest that children are consistent in the type of aggression they use over time, whether it be indirect or physical.

In the prevention of physical aggression, possible etiological links with language development are rarely taken into account. Indeed, little is known about when language and aggressive behavior become linked during development and which mechanisms are responsible for this association. This study investigated the association between physical aggression and language in late infancy with a genetic design that involved 562 19-month-old twins. A modest but significant correlation (r = -.20) was found between physical aggression and expressive vocabulary. Substantial heritability was found for physical aggression. Quantitative genetic modeling suggests that the correlation between expressive vocabulary and physical aggression cannot be explained by shared etiologies. However, phenotype-to-phenotype models indicate that the covariation can be entirely accounted for by a significant phenotypic path from expressive vocabulary to physical aggression. The implications of these results for early prevention of chronic physical aggression are discussed.

Classroom placement of twins is an ongoing issue for educational policy. Many educational jurisdictions have standard policy most commonly founded in the belief that separation supports individual identity, personal development and academic opportunity. This study examined the effects of classroom placement in a sample of 560 twin pairs whose behaviors were assessed from ages 5 to 12 years. We found no detrimental effect of classroom sharing on twins' social development. In contrast, this study provides evidence that educating twins together is associated with modest positive twins' behaviors and social functioning at school. Implications for educational policies are further discussed.

The present study investigated prenatal and early postnatal risk factors associated with developmental patterns of disruptive behaviors (DBs; e.g., hyperactivity-impulsivity, noncompliance, physical aggression) from ages 1.5 to 5 years in a population birth cohort (N = 2,057; 50.7% boys). Six high-trajectory classes obtained by latent growth modeling were used as longitudinal indicators of single-DB and co-occurrent DBs. Children following low or moderate trajectories for all DBs served as the reference class. Results showed low commonality of risk factors among single-DB trajectory classes, suggesting that "pure" forms of DBs have specific etiologies. In contrast, the trajectory classes with a high DB in common shared 20.0% to 46.7% of their risk factors. Overall, 40.0% of significant risk factors across trajectory classes were common to between two and four classes, whereas 60.0% of the significant risk factors were specific to one class or another. However, risk factors common among classes accounted for the greater part (63.2%) of the associations, especially in co-occurrent DBs trajectory classes. These risk factors included male sex, a higher number of siblings, maternal symptoms of depression and conduct problems, young motherhood, lack of positive parenting, family dysfunction, and lower socioeconomic status. Children thus develop early distinct patterns of DBs associated with both common and specific prenatal and early postnatal risk factors. Longitudinal assessments of early manifestations of DB, including a range of behaviors and a variety of potential risk factors to reflect the distinctiveness of children and their families, could help guide etiological research, tailor early interventions, and prevent a cascade of deleterious influences and outcomes.

The present study examined, within a longitudinal family-informed design and across middle childhood, the predictive associations between preference for solitude and social wariness, two forms of social withdrawal, and peer difficulties. Specifically, preference for solitude, rather than social wariness, was expected to predict peer victimization and rejection, two aspects of peer difficulties. A total of 1,014 children from the Quebec Newborn Twin Study were assessed by teachers and peers at ages 6, 7, and 10 years. Multilevel analyses conducted across three levels, between family, within family, and within person, revealed that preference for solitude, rather than social wariness, increased the risk for peer difficulties in terms of both peer victimization and peer rejection. Specifically, preference for solitude was systematically associated with peer rejection starting at age 6 years and became progressively associated with peer victimization over time. This pattern was found both between and within families. In addition, the predictive association with peer rejection was found within genetically identical, monozygotic twin pairs, suggesting that this predictive association existed after taking into account genetic vulnerabilities. Social wariness was systematically unrelated to peer difficulties. These findings suggest that preference for solitude, rather than social wariness, is a risk factor for peer difficulties. They underscore the relevance of distinguishing these dimensions of social withdrawal and illustrate the usefulness of a family-informed design to document the processes underlying childhood social adjustment.

This study aimed to identify joint trajectories of peer cyber and traditional victimization from ages 13 to 17 and individual, family, peer, and school risk factors associated with group membership. The sample was composed of 1,194 adolescents (54.2% girls). Cyber and traditional victimization were assessed at ages 13, 15, and 17. The results first revealed a low/increasing and a high/decreasing trajectories for cybervictimization and a low/decreasing and a moderate/chronic for traditional victimization. Conditional probabilities suggested that cybervictims had a high probability of being victims on school grounds, whereas traditional victims were not necessarily the target of cybervictimization. Four joint trajectory groups were also identified. With the low victimization group as the reference category, the results revealed that different sets of predictors were associated with membership in the three other joint trajectory groups. The results are discussed in relation to intervention and prevention strategies.

Poverty is a well-established risk factor for behavior problems, yet our understanding of putative family mediators during early childhood (i.e., before age 5 years) is limited. The present study investigated whether the association between poverty and behavior problems during early childhood is mediated simultaneously by perceived parenting, family dysfunction and/or maternal depression symptoms. Outcomes measures were high trajectories of physical aggression and hyperactivity between 1.5 and 5 years. Poverty was defined as living 2-4 years below the low-income thresholds defined by Statistics Canada. Using data from the first five rounds of the Quebec Longitudinal Study of Child Development, logistic regressions models showed that poverty was associated with a higher likelihood of being assigned to the high trajectory of physical aggression and hyperactivity. Overprotection and maternal depression symptoms mediated observed associations. Interventions targeting maternal depression, parenting, and poverty should help reducing children's risk for early behavior problems.

Using a sample of 767 children (403 girls, 364 boys), this study aimed to (a) identify groups with distinct trajectories of peer victimization over a 6-year period from primary school through the transition to secondary school, and (b) examine the associated personal (i.e., aggression or internalizing problems) and familial (family status, socioeconomic status, the parent-child relationship) predictors. Peer victimization was assessed via self-reports from Grades 4 through 9 (ages 10 through 15 years), aggression and internalizing problems were assessed in Grade 4 via peer nominations, and the parent-child relationship was assessed in Grade 7 (i.e., right after the transition to secondary school) via parent-reports. Growth Mixture modeling revealed 1 group (62%) who experienced little victimization in primary school and even less in secondary school, another group (31%) who was victimized in primary but not or much less in secondary school, and a third group (7%) who was chronically victimized in both school contexts. Boys were more likely than girls to follow any elevated victimization trajectory. Chronic victimization across primary and secondary school was predicted by nonintact family status and a combination of both internalizing problems and aggression compared with nonvictimized youth. In contrast, transitory victimization during primary but not in secondary school was predicted by aggression, but not internalizing problems. Support as well as conflict in the parent-child relationship also showed significant, albeit distinct associations with the different peer victimization trajectories.

Exposure to deviant friends has been found to be a powerful source of influence on children's and adolescents' aggressive behavior. However, the contribution of deviant friends may have been overestimated because of a possible non-accounted gene-environment correlation (rGE). In this study, we used a cross-lagged design to test whether friends' physical aggression at age 10 predicts an increase in participants' physical aggression from age 10 to age 13 years. Participants were 201 pairs of monozygotic twins who are part of the Quebec Longitudinal Twin Study. We performed two sets of analyses. In the first set of analyses, using twins as singletons, we found that teacher-rated friends' physical aggression predicted an increase in each twin's self-reported physical aggression from age 10 to age 13, above and beyond auto-regressive and concurrent links. Second, we used within-pair differences in regard to friends' physical aggression to predict an increase in within-pair differences in physical aggression, thus accounting for family-wide influences, including a likely rGE at age 10. No significant association was found, however. These results suggest that part of the influence attributed to friends in past studies may have been due to common underlying genetic effects on both physical aggression and association with physically aggressive friends.

Several authors consider high and frequent conflicts between friends during childhood as a serious risk for subsequent conduct problems such as generalized physical aggression toward others (e.g., Kupersmidt, Burchinal, & Patterson, 1995; Sebanc, 2003). Although it seems logical to assume that friendship conflict could have some negative consequences on children's behaviors, some scholars have suggested that a certain amount of conflict between friends may actually promote social adjustment (e.g., Laursen & Pursell, 2009). The aim of this study was to investigate the role of friendship conflict in regard to the development of generalized physical aggression toward others in the early school years (i.e., from kindergarten to Grade 1), as well as the moderating role of relational (i.e., shared positive affect and dyadic conflict resolution skills) and personal (i.e., children's sex and genetic liability for aggression) characteristics in this context. The sample included 745 twins assessed through teacher, peer, child, and friend ratings in kindergarten and Grade 1. Friendship conflict in kindergarten was linearly related to an increase in boys' but not girls' generalized physical aggression. However, shared positive affect and conflict resolution skills mitigated the prospective associations between friendship conflict and generalized physical aggression. These results were independent of children's sex, genetic risk for physical aggression, and initial levels of generalized physical aggression in kindergarten. Fostering a positive relationship between friends at school entry may buffer against the risk associated with experiencing friendship conflict.

Aggressive behavior in middle childhood is at least partly explained by genetic factors. Nevertheless, estimations of simple effects ignore possible gene-environment interactions (G x E) or gene-environment correlations (rGE) in the etiology of aggression. The present study aimed to simultaneously test for G x E and rGE processes between aggression, on the one hand, and peer victimization and the teacher-child relationship in school, on the other hand. The sample comprised 124 MZ pairs and 93 DZ pairs assessed in Grade 1 (mean age = 84.7 months). Consistent with rGE, children with a presumed genetic disposition for aggression were at an increased risk of peer victimization, whereas in line with G x E, a positive relationship with the teacher mitigated the genetically mediated expression of aggression.

The present study examined the association between theory of mind and indirect versus physical aggression, as well as the potential moderating role of prosocial behavior in this context. Participants were 399 twins and singletons drawn from two longitudinal studies in Canada. At five years of age, children completed a theory of mind task and a receptive vocabulary task. A year later, teachers evaluated children's indirect and physical aggression and prosocial behavior. Indirect aggression was significantly and positively associated with theory of mind skills, but only in children with average or low levels of prosocial behavior. Physical aggression was negatively associated with prosocial behavior but not with theory of mind. Each analysis included gender, receptive vocabulary, and the respective other subtype of aggression as control variables. These results did not differ between girls and boys or between twins and singletons. Theoretical and clinical implications of these findings are discussed.

This study investigated the evolving relations between peer victimization and social/emotional difficulties in middle to late childhood. Peer assessments of peer victimization and social/emotional difficulties (aggressive behavior, social withdrawal, and emotional vulnerability) were collected over 4 years for 1,035 children attending Grades 3-6 and were analyzed via cross-lagged panels and trajectory analyses. All dimensions were highly stable and significantly related initially. Peer victimization became progressively less related to aggression and increasingly associated with withdrawal. Bidirectional contributions over 1-year periods were found between withdrawal and emotional vulnerability and victimization. Trajectory analyses revealed heterogeneity in peer victimization patterns, with a small group of children (4.5%) being extremely victimized and with another group (10%), less severely, but increasingly victimized over time. Compared to nonvictimized children, these children were generally more emotional and became less aggressive but more socially withdrawn over time. These small behavioral changes were not associated with decreases in victimization. (Contains 5 figures and 1 table.)

Based on a sample of 406 seven-year-old twins, this study examined whether exposure to friends' social or physical aggression, respectively, moderates the effect of heritability on children's own social and physical aggression. Univariate analyses showed that children's own social and physical aggression were significantly explained by genetic factors, whereas friends' social and physical aggression represented "true" environmental factors that were unrelated to children's genetic dispositions. Multivariate analyses further suggested a possible gene-environment interaction in the link between friends' and children's physical aggression but not in the link between friends' and children's social aggression. Instead, friends' social aggression was directly related to children's social aggression, in addition to genetic effects on this behavior. Theoretical and practical implications of these results are discussed.

This study examined (a) the predictive link between peer victimization and children's reactive and proactive aggression, and (b) the potential moderating effect of reciprocal friends' reactive and proactive aggression in this context. The study also examined whether these potential moderating effects of friends' characteristics were stronger with respect to more recent friends compared to previous friends. Based on a convenience sample of 658 twin children (326 boys and 332 girls) assessed in kindergarten and first grade, the results showed that peer victimization uniquely predicted an increase in children's teacher-rated reactive aggression, but not teacher-rated proactive aggression. The relation of peer victimization to increased reactive aggression was, however, moderated by recent--not previous--reciprocal friends' similarly aggressive characteristics. These findings, however, tended to be mostly true for boys, but not for girls. The findings are discussed in terms of their theoretical and practical implications for victimized children's risk of displaying reactive and proactive aggressive behaviors.

Objective: To examine whether kindergarten children's genetic liability to physically aggress moderates the contribution of friends' aggression to their aggressive behaviors. Method: Teacher and peer reports of aggression were available for 359 6-year-old twin pairs (145 MZ, 212 DZ) as well as teacher and peer reports of aggression of the two best friends of each twin. Children's genetic risk for aggression was based on their cotwin's aggression status and the pair's zygosity. Results: Children's aggression was highly heritable. Unique environment accounted for most of the variance in friends' aggression, although there was also a small genetic contribution (15%). Both genetic liability to aggression and having aggressive friends predicted twins' aggression. However, the contribution of aggressive friends to children's aggression was strongest among genetically vulnerable children. This result was similar for boys and girls, despite sex differences in both aggression and the level of aggression of friends. Conclusions: Affiliation with aggressive friends at school entry is a significant environmental risk factor for aggression, especially for children genetically at risk for aggressive behaviors. Developmental models of aggression need to take into account both genetic liability and environmental factors in multiple settings, such as the peer context, to more precisely describe and understand the various developmental pathways to aggression. The implications for early prevention programs are discussed. (Contains 2 tables and 1 figure.)

This study investigated associations between maternal prenatal smoking and physical aggression (PA), hyperactivity-impulsivity (HI) and co-occurring PA and HI between ages 17 and 42 months in a population sample of children born in Quebec (Canada) in 1997/1998 (N=1745). Trajectory model estimation showed three distinct developmental patterns for PA and four for HI. Multinomial regression analyses showed that prenatal smoking significantly predicted children's likelihood to follow different PA trajectories beyond the effects of other perinatal factors, parental psychopathology, family functioning and parenting, and socio-economic factors. However, prenatal smoking was not a significant predictor of HI in a model with the same control variables. Further multinomial regression analyses showed that, together with gender, presence of siblings and maternal hostile reactive parenting, prenatal smoking independently predicted co-occurring high PA and high HI compared to low levels of both behaviors, to high PA alone, and to high HI alone. These results show that maternal prenatal smoking predicts multiple behavior regulation problems in early childhood.

This study compared the contribution of genes and environment to teacher-rated reactive and proactive aggression in 6-year-old twin pairs (172 pairs: 55 monozygotic girls, 48 monozygotic boys, 33 dizygotic girls, 36 dizygotic boys). Genetic effects accounted for 39% of the variance of reactive aggression and for 41% of the variance of proactive aggression. The remainder of the variance was explained by unique environmental effects. Genetic as well as unique environmental effects were significantly correlated across reactive and proactive aggression (genetic correlation = 0.87, environmental correlation = 0.34), but this overlap was largely due to a common underlying form of aggression (i.e., teacher-rated physical aggression). Once common etiological factors due to physical aggression were accounted for, reactive and proactive aggression shared no other genes and only a few environmental influences, although additional specific genetic and environmental effects were observed for both reactive and proactive aggression. These specific effects indicate that both reactive and proactive aggression may be influenced mostly by socialization experiences that are specific to each type of aggression and only to a very small degree by specific genes.

The goal of this study was to examine the links between difficult temperament (i.e., negative emotionality) and harsh parental discipline during toddlerhood, and reactive and proactive aggression in kindergarten. These links were assessed on a longitudinal population-based study of 1516 boys and girls followed longitudinally from the age of 17 months through the age of 72 months. Two possible models were tested to examine the interplay between negative emotionality and harsh parenting in predicting later reactive aggression compared to proactive aggression. The first was an additive model where both aspects make unique contributions in predicting later reactive aggression. The second model was an interactive model where harsh parenting exacerbates the link between negative emotionality and reactive aggression. Results showed a specific contribution of negative emotionality to reactive aggression. The results relative to harsh parenting are more mixed but nonetheless in line with developmental models stressing different pathways to reactive and proactive aggression.

Based on the notion that friendship may serve an important protective function against peer victimization, this study examined the moderating effect of reciprocal friends' prosociality on the link between a child's reactive aggression and victimization. The study also investigated whether a similar moderating effect could be found with respect to sibling's prosociality, given that sibling relationships have been found to provide social benefits comparable to friendships. These questions were addressed using a sample of 246 six-year-old twin pairs (246 boys and 246 girls). The results showed that a child's own reactive aggression uniquely contributed to the risk of victimization for both boys and girls. The link between reactive aggression and victimization was, however, moderated by reciprocal friends' prosocial behavior and siblings' prosocial behavior, respectively. The results are discussed in terms of their theoretical and prevention-related implications for children at risk for peer victimization.

Used a genetic design to investigate association between physical aggression and language development in 19-month-old twins. Found a modest but significant correlation between aggression and expressive vocabulary. Substantial heritability was found for physical aggression. Quantitative genetic modeling suggested that the correlation could not be explained by shared etiologies. Phenotype-to-phenotype models indicated that the covariation could be accounted for by a significant phenotypic path from expressive vocabulary to physical aggression. (Author/KB)

Evaluated contributions of birth status (gestational age and birthweight) and family adversity at birth and at age 7 to explain behavior problems at age 7. Found significant relationships between birth status and isolation, social withdrawal problems, social immaturity, and inattention. Family adversity at birth contributed to aggression reported by three sources whereas, at age 7, this index related only to parent-reported aggression and social immaturity. (Author/KB)