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Purpose: The goal of this study was to investigate the etiologic basis for the association between deficits in phonological memory (PM) and vocabulary in school-age children. Method: Children with deficits in PM or vocabulary were identified within the International Longitudinal Twin Study (ILTS; Samuelsson et al., 2005). The ILTS includes 1,045 twin pairs (between the ages of 5 and 8 years) from the United States, Australia, and Scandinavia. The authors applied the DeFries-Fulker (DeFries & Fulker, 1985, 1988) regression method to determine whether problems in PM and vocabulary tend to co-occur because of overlapping genes, overlapping environmental risk factors, or both. Results: Among children with isolated PM deficits, the authors found significant bivariate heritability of PM and vocabulary weaknesses both within and across time. However, when probands were selected for a vocabulary deficit, there was no evidence for bivariate heritability. In this case, it appears that the PM-vocabulary relationship is caused by common shared environmental experiences. Conclusions: The findings are consistent with previous research on the heritability of specific language impairment and suggest that there are etiologic subgroups of children with low vocabulary for different reasons, 1 being more influenced by genes and another being more influenced by environment. [ABSTRACT FROM AUTHOR]

We present one of the first behavior-genetic studies of individual differences in school students' levels of achievement in instructed second language acquisition (ISLA). We assessed these language abilities in Australian twin pairs (maximum N pairs = 251) by means of teacher ratings, class rankings, and self-ratings of proficiency, and used the classic twin design to estimate the relative influences of genes, shared (family/school) environment, and unique environment. Achievement in ISLA was more influenced by additive genetic effects (72%, 68%, and 38% for teacher ratings, class rankings, and twin self-ratings, respectively) than by shared environment effects, which were generally not substantial (20%, 07%, and 13%). Genetic effects distinct to speaking and listening, on the one hand, and reading and writing, on the other, were evident for the twin self-ratings. We discuss the limitations and implications of these findings and point to research questions that could profitably be addressed in future studies. [ABSTRACT FROM AUTHOR]

Two major challenges to conducting a community-based twin study of pathological gambling (PG) disorder are that: (a) it is relatively rare, and (b) individuals with the disorder in the community may be difficult to locate and recruit. We describe a new study of 4,764 individuals recruited from the Australian Twin Registry in which we attempt to effectively deal with the first challenge and examine the impact of the second challenge. The lifetime prevalence of DSM-IV PG in this Australian twin sample was 2.2%, which is 400-500% higher than has been obtained in prevalence surveys conducted in the United States. A number of predictors of non-participation were identified, including a lifetime PG disorder diagnosis, but these did not have a large net effect on the estimated number of individuals with PG or related characteristics in the twin sample. Results of biometric modeling suggested that the effect of genetic, shared family environmental, and nonshared environmental influences on the propensity to engage in 11 different specific forms of gambling (e.g., playing the lottery, betting on horse or dog races, playing electronic gaming machines) were generally moderate, low, and moderate, respectively, with mean parameter estimates obtained of 43%, 10%, and 46%. An intriguing comparison with results from a 1963 US adolescent twin study conducted by Loehlin and Nichols (1976) suggests that: (a) propensity genes for gambling involvement may be more likely to be expressed in the heavy-gambling Australian culture, or that (b) the family environment has a transient effect on the gambling behavior of young people. [ABSTRACT FROM AUTHOR]

A genetic factor model is introduced for decomposition of group differences of the means of phenotypic behavior as well as individual differences when the research variables under consideration are ordered categorical. The model employs the general Genetic Factor Model proposed by Neale and Cardon (Methodology for genetic studies of twins and families, 1992) and, more specifically, the extension proposed by Dolan et al. (Behav Genet 22: 319–335, 1992) which enables decomposition of group differences of the means associated with genetic and environmental factors. Using a latent response variable (LRV) formulation (Muthén and Asparouhov, Latent variable analysis with categorical outcomes: multiple-group and growth modeling in Mplus. Mplus web notes: No. 4, Version 5, 2002), proportional differences of response categories between groups are modeled within the genetic factor model in terms of the distributional differences of latent response variables assumed to underlie the observed ordered categorical variables. Use of the proposed model is illustrated using a measure of conservatism in the data collected from the Australian Twin Registry. [ABSTRACT FROM AUTHOR]

Abstract: Evaluation of the pattern of smoking prevalence over time could provide insight for disease prevention and tobacco control policy. Examination of assortative mating is important in assessing the relative contribution of genetic and environmental influences in behavior genetics. A significant spousal concordance for smoking could imply assortative mating behavior and have potential influence on the risk for offspring smoking. Using intergenerational data from large scale adult twin panels in Australia, the present study examined the patterns of prevalence and spousal concordance of lifetime smoking across a wide range of birth cohorts. Data were collected for more than 16,000 twins, their spouses, parents, and parents-reported grandparents. The measurement models were used to calculate prevalence and spousal concordance for smoking. Prevalence of lifetime smoking for females increased significantly over successive cohorts, and was higher for twins (0.28–0.49) than for their parents or grandparents (0.06–0.42). Smoking prevalence was still higher in males than in females for the twins (0.49–0.69), and parental or grandparental (0.62–0.77) generations, but the gender differences largely decreased in younger cohorts. Moderate but significant spousal correlations were found across cohorts and generations, which were higher for twins (0.39–0.55) than for their parents and grandparents (0.19–0.49). Despite using a simplified smoking assessment, findings in this study demonstrated changes of lifetime smoking patterns across birth cohorts and the presence of assortative mating for smoking behavior. [Copyright &y& Elsevier]

Professor Nicholas G. Martin, from QIMR Berghofer Medical Research Institute in Brisbane, Australia, is a world leader in the effort to understand the genetic architecture underlying disordered gambling. This article pays tribute to Nick and his almost two decades of gambling research, highlighting his many strengths, ranging from the use of ingenious recruitment approaches, twin study methods, genomewide association studies, to facilitating international collaborations. [ABSTRACT FROM AUTHOR]

Cohort studies are essential for conducting large studies of multiple exposures and outcomes in humans. Recently, the ability to combine data from multiple cohorts in, for example, meta-analyses, and the willingness in the genetics community to collaborate to enable replication studies has led to many new insights into the genetic and environmental determinants of human health and behaviors. The contribution of Professor Nicholas Martin to the development of cohort studies, particularly of twin and twin-family studies, over a period of several decades is reviewed. He has contributed to the development and use of both Australian and international resources. The contributions of Australian twin studies to genomewide association projects are multiple, and across multiple domains, from biomarkers, lifestyle and behavior to disorders and disease. [ABSTRACT FROM AUTHOR]

This article describes the Australian Twin Attention-deficit/hyperactivity disorder (ADHD) Project (ATAP), the results of research conducted using this database and plans for future studies. Information has been actively collected from Australian families with twin children since 1991 for the ATAP database. The value of assessing siblings as well as twins is emphasized. Much work has gone into continuing the involvement of families in the study though this does become more difficult when twins reach maturity. The main focus of the project is ADHD in children and adolescents plus comorbid conditions including conduct disorder, oppositional defiant disorder, and generalized anxiety disorder. A major challenge has been how to retain continuity in the assessments, while at the same time covering changes in psychiatric classification, such as the move to Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV; American Psychiatric Association, 1994). Changes in the scale can affect the reports of twin similarity. Over the years, these twins have become part of other twin studies and future plans include linking different twin databases to investigate the relationships between childhood behavior and adult conditions. Recruitment, assessment and retention of twin families require a major commitment but create a significant resource for collaboration in areas outside the original aim. [ABSTRACT FROM AUTHOR]

Conclusions about the effects of harsh parenting on children have been limited by research designs that cannot control for genetic or shared environmental confounds. The present study used a sample of children of twins and a hierarchical linear modeling statistical approach to analyze the consequences of varying levels of punishment while controlling for many confounding influences. The sample of 887 twin pairs and 2,554 children came from the Australian Twin Registry. Although corporal punishment per se did not have significant associations with negative childhood outcomes, harsher forms of physical punishment did appear to have specific and significant effects. The observed association between harsh physical punishment and negative outcomes in children survived a relatively rigorous test of its causal status, thereby increasing the authors' conviction that harsh physical punishment is a serious risk factor for children. [ABSTRACT FROM AUTHOR]

Intelligence, as measured by standardized psychological tests, has been shown to be highly heritable, though identifying specific genes influencing general intelligence has proven difficult. We conducted genome-wide linkage analyses to identify chromosomal regions containing genes influencing intelligence, as measured by WAIS full-scale IQ (FSIQ), performance IQ (PIQ) and verbal IQ (VIQ). Non-parametric multipoint linkage analyses were conducted with Merlin-regress software, using a sample of 1111 genotyped and phenotyped individuals from 201 families, ascertained as part of the Collaborative Study on the Genetics of Alcoholism (COGA). The strongest evidence of linkage was obtained for FSIQ on chromosome 6 (LOD=3.28, 12 cM) near the marker D6S1006. This region was also implicated with suggestive linkage in a recently published genome screen of IQ in Australian and Dutch twin pairs, and it has been implicated in linkage studies of developmental dyslexia. Our findings provide further support that chromosome 6p contains gene(s) affecting intelligence. [ABSTRACT FROM AUTHOR]

Parental divorce is associated with a number of emotional and behavioral problems in young-adult offspring, but theoretical and empirical considerations suggest that the relation may be partially or fully accounted for by passive gene-environment correlation or environmental selection characteristics. The current study used the Children of Twins Design to explore whether shared environmental or genetic factors confound the relationship between parental marital instability and measures of psychopathology. Comparisons of the offspring of adult twins in Australia on 3 factors of abnormal behavior, including drug and alcohol, behavioral, and internalizing problems, suggest that environmental influences associated with divorce account for the higher rates of psychopathology. The results are consistent with a causal connection between marital instability and psychopathology in young-adult offspring. [ABSTRACT FROM AUTHOR]

We estimated genetic and environmental influences on mother-rated DSM-III-R separation anxiety disorder (SAD) symptoms in 2043 3 to 18-year-old male and female twin pairs and their siblings (348 pairs) recruited from the Australian NH&MRC Twin Registry. Using DeFries and Fulker's (1985) multiple regression analysis, we found that genetic and shared environmental influences both contributed appreciably to variation in SAD symptoms (h [sup 2] = .47, SE = .07; c [sup 2] = .21, SE = .05) and were significantly moderated by both sex and age. Genetic influences were greater for girls than boys (h [sup 2] = .50 and .14, respectively), whereas shared environmental influences were greater for boys than girls (c [sup 2] = .51 and .21, respectively). Genetic influences increased with age, whereas shared environmental influences decreased with age. Shared environmental influences were greater in magnitude for twins than for nontwin siblings (c [sup 2] = .28 versus .13, respectively). Implications of these findings for theories of the cause of separation anxiety are discussed. [ABSTRACT FROM AUTHOR]

We examine the hypothesis that environmental transmission is a significant factor in individual differences for Neuroticism among 45,850 members of extended twin kinships from Australia (N = 20,945) and the United States (N = 24,905). To this large data set we fitted a model estimating genetic and environmental components of variance and gene-environmental covariance to examine the causes of individual differences in Neuroticism. For the combined sample we reject models including environmental transmission, shared environment, and a special twin environment in favor of more parsimonious genetic models. The best-fitting model involved only modest assortative mating, nonshared environment, and both additive and nonadditive genetic components. We conclude, first, that there is no evidence for environmental transmission as a contribution to individual differences in Neuroticism in these replicated samples, drawn from different continents, and, second, that a simple genetic structure underlies familial resemblance for the personality trait of Neuroticism. It is interesting that, despite the opportunity provided by the elaborate design and extensive power of our study, the picture revealed for the causes of individual differences in Neuroticism is little more complex than that found from earlier, simpler designs applied to smaller samples. However, this simplicity could not have been confirmed without using a highly informative design and a very large sample. [ABSTRACT FROM AUTHOR]

The present analyses examined genetic influences on alcohol metabolism and their possible relationship to risk of alcohol dependence. Subjects were 206 Australian twin pairs who participated in an alcohol challenge protocol in 1979–1981, in which they were given a 0.75 g/kg dose of alcohol; blood alcohol concentrations (BACs) measured at five times over a 3-hr period after alcohol ingestion were examined. Structural equation modeling, fitting a combined autoregressive and common factor model, indicated significant heritabilities for both men and women (h [sup 2] range = 0.19–0.71), with significant parameter heterogeneity as a function of gender. In 1992–1993, both twins from 159 of the alcohol challenge pairs completed a telephone-administered psychiatric diagnostic interview. Repeated-measures MANOVAs were used to examine whether respondent's or cotwin's DSM-III-R alcohol dependence status, or parental history of alcohol problems, was associated with variation in alcohol metabolism. There was some evidence that individuals at increased genetic risk of alcohol dependence [with either a paternal history of alcohol problems (women) or an MZ male cotwin who reported a history of alcohol dependence by 1992–1993] showed lower initial BACs than other groups. However, this effect was not seen in those who themselves had a history of alcohol dependence by interview follow-up, perhaps because this relationship was already masked by a history of excessive drinking at baseline. [ABSTRACT FROM AUTHOR]

Retrospective data on age at onset of smoking, reported by 3810 adult Australian twin pairs, were analyzed to determine the role of genetic and environmental factors in the onset of smoking. Results of nonmetric multidimensional scaling supported a two-process model in which different etiologic factors determined which individuals were at risk of becoming smokers and the age at onset of smoking in those who were at risk. Parametric model-fitting confirmed this difference. For female twins and younger male twins (aged 30 years or less), the onset of smoking was strongly influenced by genetic factors, with shared and nonshared environmental effects having a more modest impact. For older male twins, shared environmental influences on onset of smoking were very important, and the influence of genetic predisposition was slight. The age at which smoking onset occurred, however, was influenced by both genetic and nonshared environmental effects, but not by shared environmental effects, in both sexes and both cohorts. [ABSTRACT FROM AUTHOR]

A universal problem in genetic association studies is to distinguish associations due to genuine effects of the locus under investigation, or linkage disequilibrium with a nearby locus that has a genuine effect, from associations due to population stratification or other artifacts. Fulker et al. (1999) have suggested a test using unselected sib pairs to distinguish these two causes of association. The test is readily implemented within a standard maximum-likelihood framework using the Mx package. The approach is applied to data on ADH2 genotypes and a measure of alcohol consumption from an Australian DZ twin pair sample. Results indicate that the association of the ADH2 *2 allele with lower alcohol consumption cannot be explained by simple admixture and that there may be genuine allelic effects of the locus on alcohol consumption. Power calculations are provided to show that these results are plausible for the sample size in this study and consider the effects of genetic architecture and sample structure on required sample sizes for the Fulker et al. test. [ABSTRACT FROM AUTHOR]

Although attention-deficit/hyperactivity disorder (ADHD) is highly heritable, emerging evidence suggests symptoms are associated with interactions between genes and the environment (GxE) during development. This study tested whether heritability of ADHD symptoms is moderated by two environmental factors: socioeconomic status (SES) and chaos (household disorganisation). A population sample of 520 twin pairs (N = 1040, 52.3% female) from 6 to 15 years completed measures of behavior and home environment. Structural equation modelling was then used to test whether environmental factors were associated with a change in the extent to which genes explain variability in ADHD symptoms. Neither chaos nor SES moderated heritability, with consistent contributions from both genes and environment indicated across socioeconomic strata and levels of chaos. This finding contrasts with those of previous research, underlining the need to replicate results in the emerging field of GxE research across different populations and statistical methods. Robust findings may assist in developing targeted interventions for genetically vulnerable individuals.

With the development of large twin-family registers worldwide (see the December 2006 issue of Twin Research and Human Genetics for an overview) new options for research projects become feasible, including longitudinal studies. In this special issue on Longitudinal Twin and Family Studies, several aspects of longitudinal research are highlighted and an overview of results is given from European, Australian and United States twin studies. These studies deal with a time span from birth to adulthood and a wide range of phenotypes is covered. Data collection in some studies is age/cohort driven; in others, subjects are followed from different ages at regular time intervals. Most studies are based on data collection in twins, but adoption and parent–offspring data designs are described as well. The overview of results of current longitudinal projects makes this special issue a reference resource for longitudinal research in developmental behavior genetics. [ABSTRACT FROM AUTHOR]

The present study investigated the extent to which individual differences in humor styles are attributable to genetic and/or environmental factors in an Australian sample. Participants were 934 same-sex pairs of adult twins from the Australian Twin Registry (546 monozygotic pairs, 388 dizygotic pairs) who completed the Humor Styles Questionnaire (HSQ). The HSQ measures four distinct styles of humor – affiliative, self-enhancing, aggressive, and self-defeating. Results revealed that additive genetic and non-shared environmental factors accounted for the variance in all four humor styles, thus replicating results previously obtained in a sample of twins from the United Kingdom. However, a study conducted with a U.S. sample produced different results and we interpret these findings in terms of cross-cultural differences in humor. [ABSTRACT FROM AUTHOR]