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The seasonality of demographic data has been of great interest. It depends mainly on the climatic conditions, and the findings may vary from study to study. Commonly, the studies are based on monthly data. The population at risk plays a central role. For births or deaths over short periods, the population at risk is proportional to the lengths of the months. Hence, one must analyze the number of births (and deaths) per day. If one studies the seasonality of multiple maternities, the population at risk is the total monthly number of confinements and the number of multiple maternities in a given month must be compared with the monthly number of all maternities. Consequently, when one considers the monthly rates of multiple maternities, the monthly number of births is eliminated and one obtains an unaffected seasonality measure of the rates. In general, comparisons between the seasonality of different data sets presuppose standardization of the data to indices with common means, mainly 100. If one assumes seasonality as 'non-flatness' throughout a year, a chi-squared test would be an option, but this test calculates only the heterogeneity and the same test statistic can be obtained for data sets with extreme values occurring in consecutive months or in separate months. Hence, chi-squared tests for seasonality are weak because of this arbitrariness and cannot be considered a model test. When seasonal models are applied, one must pay special attention to how well the applied model fits the data. If the goodness of fit is poor, nonsignificant models obtained can erroneously lead to statements that the seasonality is slight, although the observed seasonal fluctuations are marked. In this study, we investigate how the application of seasonal models can be applied to different demographic variables. [ABSTRACT FROM AUTHOR]

We investigated genetic and environmental correlations and gene by environment interactions (GxE) between depressive symptoms measured by the Beck Depression Inventory (BDI) and quantity smoked measured by number of cigarettes smoked per day (CPD) using quantitative genetic modeling. The population-based sample consisted of 12,063 twin individuals from the Finnish Twin Cohort Study. Bivariate Cholesky decomposition revealed that the phenotypic correlation (r = 0.09) between BDI and CPD was explained by shared genetic (r g = 0.18) and environmental (r e = 0.08) factors. GxE models incorporating moderator effects were built by using CPD as trait and BDI as moderator and vice versa. The importance of the genetic variance component increased with increasing moderator value in both models. Thus, the influence of genetic effects on variance of smoking quantity was enhanced in individuals with elevated depression score and vice versa; the genetic effects on depression variance were potentiated among heavy smokers. In conclusion, shared genetic and environmental factors as well as GxE underlie the association of smoking with depression. [ABSTRACT FROM AUTHOR]

Studies have suggested both adverse and protective associations of obesity with depressive symptoms. We examined the contribution of environmental and heritable factors in this association. Participants were same-sex twin pairs from two population-based twin cohort studies, the Older Finnish Twin Cohort (n = 8,215; mean age = 44.1) and the US Midlife Development in the United States (MIDUS; n = 1,105; mean age = 45.1). Body mass index (BMI) was calculated from self-reported height and weight. Depressive symptoms were assessed using Beck's Depression Inventory (BDI; Finnish Twin Cohort), and by negative and positive affect scales (MIDUS). In the Finnish Twin Cohort, higher BMI was associated with higher depressive symptoms in monozygotic (MZ) twins (B = 2.01, 95% CI = 1.0, 3.0) and dizygotic (DZ) twins (B = 1.17, 0.5, 1.9) with BMI >22. This association was observed in within-pair analysis in DZ twins (B = 1.47, CI = 0.4, 2.6) but not in within-pair analysis of MZ twins (B = 0.03, CI = -1.9, 2.0). Consistent with the latter result, a bivariate genetic model indicated that the association between higher BMI and higher depressive symptoms was largely mediated by genetic factors. The results of twin-pair analysis and bivariate genetic model were replicated in the MIDUS sample. These findings suggest an association between obesity and higher depressive symptoms, which is largely explained by shared heritable biological mechanisms. [ABSTRACT FROM AUTHOR]

Pregnancy- and birth-related factors may have an effect on handedness. Compared with singletons, twins have a lower birth weight, shorter gestational age, and are at higher risk for birth complications. We tested whether the prevalence of left-handedness is higher among twins than singletons, and if so, whether that difference is fully explained by pregnancy and birth-related differences between twins and singletons. We analyzed Finnish population-based datasets; included were 8,786 twins and 5,892 singletons with information on birth weight (n = 12,381), Apgar scores (n = 11,129), and gestational age (n = 11,811). Two twin cohorts were involved: FinnTwin12 included twins born during 1983–1987, and FinnTwin16 included twins born during 1974–1979. We had two comparison groups of singletons: 4,101 individuals born during 1986–1988 and enrolled in the Helsinki Ultrasound Trial, and 1,791 individuals who were partners of FinnTwin16 twins. We used logistic regression models with writing hand as the outcome for comparison and evaluating effects of covariates. Left-handedness was more common in twins (9.67%) than in singletons (8.27%; p = .004). However, Apgar scores were associated with handedness, and after controlling for covariates, we found no difference in the prevalence of left-handedness between twins and singletons. Increased left-handedness among twins, often reported by others, was evident in our data, but only among our older twin cohorts, and that association disappeared after removing effects of perinatal covariates. [ABSTRACT FROM PUBLISHER]

We analyzed the association between mean height and old age cognition in two Nordic twin cohorts with different childhood living conditions. The cognitive performance of 4720 twin individuals from Denmark (mean age 81.6 years, SD = 4.59) and Finland (mean age 74.4 years, SD = 5.26) was measured using validated cognitive screens. Taller height was associated with better cognitive performance in Finland (β-estimates 0.18 SD/10cm, p value < .001, for men and 0.13 SD, p = .008, for women), but this association was not significant in Denmark (β-estimates 0.0093 SD, p value = .16, for men and 0.0075 SD, p value = .016, for women) when adjusted for age and education/social class. Among Finnish participants higher variability of cognitive performance within shorter height quintiles was observed. Analysis using gene-environment interaction models showed that environmental factors exerted a greater impact on cognitive performance in shorter participants, whereas in taller participants' it was explained mainly by genetic factors. Our results suggest that shorter participants with childhood adversity are more vulnerable to environmental risk factors for cognitive impairment. [ABSTRACT FROM PUBLISHER]

We analyzed depressive and psychosomatic symptoms in relation to co-twin dependence in 419 twins at the age of 22 to 30 years. Depressive symptoms were assessed, as previously, with Children's Depression Inventory modified to be suitable for this age and reported as a total score and three subscales (low self-confidence, anhedonia and sadness) based on factor analysis as reported in a previous epidemiological study conducted in Finland. Items assessing nervous complaints and somatic symptoms were adapted from Finnish studies of juvenile health habits. Inter-twin dependence decreased with increasing age in both genders. Monozygotic twins, especially monozygotic females, reported most often to be dependent on their co-twin. When the symptom reporting was evaluated in relation to co-twin dependence, no relation was found between co-twin dependence and depressive symptom reporting. However, dependence-independence imbalance within twin pair was associated with elevated levels of depressive and psychosomatic symptoms, especially in twins who perceived themselves as dependent and the co-twin as independent. We conclude that there was no relation found between co-twin dependence and depressive symptom reporting in male and female twins except for the few imbalance cases, where most symptoms were reported by those dependent twins who felt their co-twin as independent. [ABSTRACT FROM AUTHOR]

We examined whether externalizing problem behaviors (hyperactivity-impulsivity, aggressiveness, and inattention) predict illicit drug use independently, or whether their associations with drug use are mediated through cigarette smoking. We used a prospective longitudinal design within the FinnTwin12-17 study among Finnish adolescents with baseline at age 12 and follow-up surveys at ages 14 and 17. Path models were conducted with Mplus and included 1992 boys and 2123 girls. The outcome was self-reported ever use of cannabis or other illicit drugs at age 17. The predictors were: externalizing behaviors (hyperactivity-impulsivity, aggressiveness, and inattention) assessed by teachers and parents (age 12) and self-reported cigarette smoking (age 14). The findings differed across behavior studied. The association of hyperactivity-impulsivity with drug use was mostly mediated through earlier cigarette smoking. Concerning aggressiveness and inattention, the results were different among girls than boys. Among girls no significant mediation occurred, whereas among boys more consistent evidence on mediation was seen. Consistently in all models, the direct association of early cigarette smoking on drug use was strong and highly significant. We conclude that the associations of externalizing problem behaviors with illicit drug use are partially mediated through cigarette smoking. Although interventions targeting externalizing problem behaviors may protect adolescents from early onset smoking and subsequently experimenting with drugs, interventions to prevent cigarette smoking initiation are also important in reducing risk of later drug use. [ABSTRACT FROM AUTHOR]

Probably the most robust sex difference in cognitive abilities is that on average males outperform females in tests of mental rotation. Using twin data we tested whether there are sex differences in the magnitude of genetic and environmental effects on mental rotation test performance and whether the same or different genetic effects operate in females and males. The present study replicated the well-known male advantage in mental rotation ability. The relative proportion of variance explained by genetic effects did not differ between females and males, but interestingly, absolute additive genetic and unique environmental variances were greater in males reflecting significantly greater phenotypic variance in mental rotation test performance in males. Over half of the variance in mental rotation test performance was explained by genetic effects, which suggest that mental rotation ability is a good phenotype for studies finding genes underlying spatial abilities. Results indicate that females and males could be combined for such genetic studies, because the same genetic effects affected mental rotation test performance in females and males. [ABSTRACT FROM AUTHOR]

Bread is an elementary part of the western diet, and especially rye bread is regarded as an important source of fibre. We investigated the heritability of eating bread in terms of choice of white and rye bread and use-frequency of bread in female and male twins in Denmark and Finland. The study cohorts included 575 Danish (age range 18-67 years) and 2009 Finnish (age range 22-27 years) adult twin pairs. Self-reported frequency of eating bread was obtained by food frequency questionnaires. Univariate models based on linear structural equations for twin data were used to estimate the relative magnitude of the additive genetic, shared environmental and individual environmental effects on bread eating frequency and choice of bread. The analysis of bread intake frequency demonstrated moderate heritability ranging from 37-40% in the Finnish cohort and 23-26% in the Danish cohort. The genetic influence on intake of white bread was moderate (24-31%), while the genetic influence on intake of rye bread was higher in men (41-45%) than in women (24-33%). Environmental influences shared by the twins were not significant. Consumption of bread as well as choice of bread is influenced by genetic predisposition. Environmental factors shared by the co-twins (e.g., childhood environment) seem to have no significant effects on bread consumption and preference in adulthood. [ABSTRACT FROM AUTHOR]

The aim of this study was to examine whether maximal walking speed, maximal isometric muscle strength, leg extensor power and lower leg muscle cross-sectional area (CSA) shared a genetic effect in common. In addition, we wanted to identify the chromosomal areas linked to maximal walking speed and these muscle characteristics and also investigate whether maximal walking speed and these three skeletal muscle characteristics are regulated by the same chromosomal areas. We studied 217 monozygotic (MZ) and dizygotic (DZ) female twin pairs aged 66 to 75 years in the Finnish Twin Study on Aging study. The DZ pairs (94) were genotyped for 397 microsatellite markers in 22 autosomes and X-chromosome. Genetic modeling showed that, muscle CSA, strength, power and walking speed shared a genetic effect in common which accounted for 7% of the variation in CSA, 51% in strength, 37% in power and 35% in walking speed. The results of an explorative multipoint linkage analysis suggested that the highest LOD score found for each phenotype was 2.41 for walking speed on chromosome 13q22.1, 2.14 for strength on chromosome 15q14, 2.84 for power on chromosome 8q24.23, and 2.93 for muscle CSA on chromosome 20q13.31. Also a suggestive LOD score, 2.68, for muscle CSA was found on chromosome 9q34.3. The chromosomal areas of a suggestive linkage for strength and power partly overlapped LOD scores higher than 1.0 being seen for these phenotypes on chromosome 15. The present study was the first genome-wide linkage analysis to be conducted for these multifactorial and clinically important phenotypes underlying functional independence in older women. [ABSTRACT FROM AUTHOR]

We enrolled more than 3500 same-sex twinsfrom 5 consecutive Finnish birth cohorts into a longitudinal study as each cohort reached age 16. Twins completed the Psychopathic Deviate (Pd) Scale of the Minnesota Multiphasic Personality Inventory at baseline, Sensation Seeking Scale items as each cohort reached age 17, and later, at average ages 18.5 and 25, the Rutgers Alcohol Problem Index (RAPI). Using raw maximum likelihood estimation, we fit a Cholesky model to the 4 variables assessed at 4 ages across the 4 twin types; we estimated genetic and environmental influences on the stability of alcohol problems across development and the genetic and environmental contributions to predictive correlations between adolescent personality and later alcohol-related behavior problems. With one exception, the phenotypic, genetic, and environmental correlations were very similar for males and females. The exception was that the lagged associations of Pd and RAPI reflect a higher genetic correlation among males than females and a higher environmental correlation among females than males. Our analyses suggest that developmental changes underlying variation in alcohol problems from late adolescence to early adulthood differ for males and females. In males, the main change is decreased variation due to shared environmental effects; the magnitude of genetic effects is stable over time, and the high genetic correlation, .95, suggests that the same genetic influences are important at both ages. Among females, in contrast, genetic influences decline in magnitude from age 18 to 25, and at least part of the genetic effect evident at age 25 differs from the genetic effect evident at age 18. [ABSTRACT FROM AUTHOR]

In studies on the Finnish Twin Cohorts, genetic and environmental determinants of common, complex diseases, and their behavioral risk factors have been investigated in Finland. In 1974, the older twins were identified, with a total of 13,888 like-sexed pairs of known zygosity. They have participated since 1975 in mail surveys, clinical examinations for subsamples, have been used to recruit families, and have been followed up for morbidity using national medical registers. Opposite twin pairs were added later. Two longitudinal studies of adolescent twins and their families, known as the FinnTwin12 and FinnTwin16 studies, have focused on determinants of health-related behaviors and disease in adolescents and young adults. This article describes current, ongoing data collection and analyses. [ABSTRACT FROM AUTHOR]