Child care, socio-economic status and problem behavior: a study of gene-environment interaction in young Dutch twins.

The influences of formal child care before age 4 on behavioral problems at 3, 5, and 7 years of age were assessed in 18,932 Dutch twins (3,878 attended formal child care). The effect of formal child care was studied on the average level of problem behavior and as moderator of genetic and non-genetic influences, while taking into account effects of sex and parental socio-economic status (SES). There was a small association between attending formal child care and higher externalizing problems, especially when SES was low. Heritability was lower for formal child care and in lower SES conditions. These effects were largest at age 7 and for externalizing problems. In 7 year-old boys and girls, the difference in heritability between the formal child care group of low SES and the home care group of high SES was 30% for externalizing and ~20% for internalizing problems. The decrease in heritability was explained by a larger influence of the environment, rather than by a decrease in genetic variance. These results support a bioecological model in which heritability is lower in circumstances associated with more problem behavior.

Effects of chorionicity and zygosity on triplet birth weight.

Birth weight in triplets is, on average, lower than in singletons and twins, and more children are classified as having very low or extremely low birth weight. Still, there is limited research on factors that affect triplet birth weight, and samples under study are often small. Chorionicity and zygosity influence triplet birth weight, but it is unknown whether the effect of zygosity can be entirely ascribed to the effect of chorionicity or whether zygosity has an additional effect on triplet birth weight. This question was investigated in 346 triplets (from 116 trios) registered with the Netherlands Twin Register for whom data on chorionicity were available. 'Triplet' refers to one child and the set of three triplets is referred to as 'trio'. Trios and triplets were classified based on zygosity and chorionicity. With regression analysis, the effects of zygosity and chorionicity on triplet birth weight were examined, while controlling for gestational age, sex, and maternal smoking during pregnancy. In addition, within the dizygotic trios a within-family comparison was made between the birth weight of the triplets that were part of a monozygotic pair (with some pairs sharing a chorion), and the birth weight of the dizygotic triplet. Based on the classification on individual level, monozygotic, monochorionic triplets had a lower mean birth weight than dizygotic, dichorionic triplets. Most remarkably, in dizygotic trios, monozygotic pairs only had a lower mean birth weight than their dizygotic sibling triplet when the pair shared a chorion. We conclude that having shared a chorion, rather than being monozygotic, increases the risk of a low birth weight.

Gene-environment interaction in teacher-rated internalizing and externalizing problem behavior in 7- to 12-year-old twins.

BACKGROUND: Internalizing and externalizing problem behavior at school can have major consequences for a child and is predictive for disorders later in life. Teacher ratings are important to assess internalizing and externalizing problems at school. Genetic epidemiological studies on teacher-rated problem behavior are relatively scarce and the reported heritability estimates differ widely. A unique feature of teacher ratings of twins is that some pairs are rated by different and others are rated by the same teacher. This offers the opportunity to assess gene-environment interaction. METHODS: Teacher ratings of 3,502 7-year-old, 3,134 10-year-old and 2,193 12-year-old twin pairs were analyzed with structural equation modeling. About 60% of the twin pairs were rated by the same teacher. Twin correlations and the heritability of internalizing and externalizing behavior were estimated, separately for pairs rated by the same and different teachers. Socioeconomic status and externalizing behavior at age 3 were included as covariates. RESULTS: Twin correlations and heritability estimates were higher when twin pairs were in the same class and rated by the same teacher than when pairs were rated by different teachers. These differences could not be explained by twin confusion or rater bias. When twins were rated by the same teacher, heritability estimates were about 70% for internalizing problems and around 80% in boys and 70% in girls for externalizing problems. When twins were rated by different teachers, heritability estimates for internalizing problems were around 30% and for externalizing problems around 50%. CONCLUSIONS: Exposure to different teachers during childhood may affect the heritability of internalizing and externalizing behavior at school. This finding points to gene-environment interaction and is important for the understanding of childhood problem behavior. In addition, it could imply an opportunity for interventions at school.

Birth weight in a large series of triplets.

BACKGROUND: Triplets are often born premature and with a low birth weight. Because the incidence of triplet births is rare, there are relatively few studies describing triplet birth weight characteristics. Earlier studies are often characterized by small sample sizes and lack information on important background variables such as zygosity. The objective of this study is to examine factors associated with birth weight in a large, population-based sample of triplets registered with the Netherlands Twin Register (NTR). METHODS: In a sample of 1230 triplets from 410 families, the effects of assisted reproductive techniques, zygosity, birth order, gestational age, sex, maternal smoking and alcohol consumption during pregnancy on birth weight were assessed. The resemblance among triplets for birth weight was estimated as a function of zygosity. Birth weight discordance within families was studied by the pair-wise difference between triplets, expressed as a percentage of the birth weight of the heaviest child. We compare data from triplets registered with the NTR with data from population records, which include live births, stillbirths and children that have deceased within days after birth. RESULTS: There was no effect of assisted reproductive techniques on triplet birth weight. At gestational age 24 to 40 weeks triplets gained on average 130 grams per week; boys weighed 110 grams more than girls and triplets of smoking mothers weighted 104 grams less than children of non-smoking mothers. Monozygotic triplets had lower birth weights than di- and trizygotic triplets and birth weight discordance was smaller in monozygotic triplets than in di- and trizygotic triplets. The correlation in birth weight among monozygotic and dizygotic triplets was 0.42 and 0.32, respectively. In nearly two-thirds of the families, the heaviest and the lightest triplet had a birth weight discordance over 15%. The NTR sample is representative for the Dutch triplet population that is still alive 28 days after birth. CONCLUSION: Birth weight is an important determinant of childhood development. Triplet status, gestational age, sex, zygosity and maternal smoking affect birth weight. The combined effects amount to a difference of 364 grams between monozygotic girl triplets of smoking mothers compared to dizygotic boy triplets of non-smoking mothers of the same gestational age. Birth weight in triplets is also influenced by genetic factors, as indicated by a larger correlation in monozygotic than in di- and trizygotic triplets.

Heritability of anxious-depressive and withdrawn behavior: age-related changes during adolescence.

OBJECTIVE: To explain the differential course of anxiety and depression in individuals from childhood to adulthood by examining age-related changes in the genetic and environmental etiology of anxious and depressive symptoms. METHOD: A sample of 1470, 1839, and 2023 Dutch twins aged 12, 14, and 16 years reported on symptoms of anxious depression (AD) and withdrawn behavior (WB), using the Youth Self Report (YSR). AD and WB were analyzed with bivariate cross-sectional genetic models for each age group to obtain estimates of the relative influence of genes (A), shared (C), and nonshared (E) environment. RESULTS: The best-fitting models revealed no difference between heritability estimates in boys and girls. Familial clustering at age 12 years was explained by genetic and shared environmental factors. At ages 14 and 16 years, genetic factors were sufficient to explain familial clustering, shared environmental effects were absent. Genetic influences on AD and WB correlated highly. CONCLUSIONS: These findings are in agreement with earlier studies on age-specific effects of genes and shared environment on anxiety, depression, and withdrawn behavior in childhood and adolescence. The current study demonstrated that the decrease in the role of shared environment occurs after age 12. Hormonal changes accompanying the onset of puberty do not seem to explain the change in risk factors, as in 90% of the subjects puberty had already started. More knowledge on age-specific risk factors may offer opportunities for therapeutic interventions.