Genetic and social influences on starting to smoke: a study of Dutch adolescent twins and their parents.

In a study of 1600 Dutch adolescent twin pairs we found that 59% of the inter-individual variation in smoking behaviour could be attributed to shared environmental influences and 31% to genetic factors. The magnitude of the genetic and environmental effects did not differ between boys and girls. However, environmental effects shared by male twins and environmental effects shared by female twins were imperfectly correlated in twins from opposite-sex pairs, indicating that different environmental factors influence smoking in adolescent boys and girls. In the parents of these twins, the correlation between husband and wife for 'currently smoking' (r = 0.43) was larger than for 'ever smoked' (r = 0.18). There was no evidence that smoking of parents (at present or in the past) encouraged smoking in their offspring. Resemblance between parents and offspring was significant but rather low and could be accounted for completely by their genetic relatedness. Moreover, the association between 'currently smoking' in the parents and smoking behaviour in their children was not larger than the association between 'ever smoking' in parents and smoking in their children.

Left-handedness in twins: genes or environment?

Twin family data can cast light on the longstanding problem about the influences of genes and environment on the etiology of left-handedness. Therefore, hand preference was assessed in 1700 adolescent twin pairs and their parents. Left-handedness (LH) appeared not significantly enhanced among twins compared to the general population. In addition the following observations were made: (1) Significant more LH in first born twins than in second born twins. (2) Significant higher left-handedness association in MZmm pairs compared to DZmm pairs and not or may be marginally so in MZff versus DZff pairs. These results, combined with the observations that (a) left-handed fathers increase the probability of LH in sons but not in daughters; (b) LH in mothers increases LH prevalence in both sons and daughters to the same degree; and (c) very low birth weight, corrected for the effect of gestational age, increases LH prevalence in first born twins only, make an environmental explanation more likely. The possibility that exposure to prenatal male hormones - to which low birth weight and high birth stress children are more vulnerable - might be a crucial condition for the etiology of LH, is discussed.

Familial resemblances in alcohol use: genetic or cultural transmission?

OBJECTIVE: Resemblances between parents and children for alcohol use can be due both to cultural transmission and genetic inheritance. We examined the genetic and environmental determinants of the familial resemblances in alcohol use. METHOD: With a parent-twin design a distinction was made between the contribution of genetic effects, the environmental influences shared by siblings and the effects of cultural transmission from parents to offspring. By questionnaire data on whether subjects had ever used alcohol were obtained from 403 Dutch families with a twin aged 15-16 years old and from 805 families with a twin aged 17 years and older. RESULTS: For 15-16 year olds, the resemblance between parents and offspring could be explained either by genetic inheritance or cultural transmission. Shared environment explained between 58% and 88% of the individual differences in adolescent alcohol use. For twins aged 17 years and older, 43% of the individual differences in alcohol use could be attributed to genetic factors and 37% to shared environment. There was no evidence for cultural transmission in this age group. CONCLUSIONS: For adolescents aged 17 years and older parental alcohol use did not create an environment that stimulated alcohol use in children. The resemblance for alcohol use between parents and their children aged 17 years and older could be explained by their genetic relatedness. For 15-16 year old adolescents, shared environmental influences were more important than for older adolescents. Only 10% of this shared environmental variance might be influenced by parental alcohol use due to cultural transmission.

A multivariate genetic analysis of sensation seeking.

The genetic architecture of sensation seeking was analyzed in 1591 adolescent twin pairs. Individual differences in sensation seeking were best explained by a simple additive genetic model. Between 48 and 63% of the total variance in sensation seeking subscales was attributable to genetic factors. There were no sex differences in the magnitude of the genetic and environmental effects. The different dimensions of sensation seeking were moderately correlated. The strongest correlations were between the subscales Thrill and Adventure Seeking and Experience Seeking (r = 0.4) and between Boredom Susceptibility and Disinhibition (r = 0.4 in males, r = 0.5 in females). A triangular decomposition showed that the correlations between the sensation seeking subscales were induced mainly by correlated genetic factors and, to a smaller extent, by correlated unique environmental factors. The genetic and environmental correlation structures differed between males and females. For females, higher genetic correlations for Experience Seeking with Boredom Susceptibility and Disinhibition and higher correlations among the unique environmental factors were found. There was no evidence that sex-specific genes influenced sensation seeking behavior in males and females.

Association between alcohol use and smoking in adolescent and young adult twins: a bivariate genetic analysis.

The association between alcohol use and smoking was examined in a large population-based sample of Dutch twins consisting of three age groups; young adolescent twins aged 12-14 years (n = 650 twin pairs), 15-16-years-old adolescent twins (n = 705 twin pairs), and young adult twins aged 17-25 years (n = 1266 twin pairs). For all three age groups, alcohol use and smoking were correlated (r = 0.5-0.6). Adolescents and young adults who smoked were more likely to drink alcohol than nonsmokers. The relation between alcohol use and smoking was also found within a twin pair; alcohol use in one twin was correlated with smoking in the cotwin. This finding suggested that familial factors contribute to the association between alcohol and tobacco use. With a bivariate genetic model, it was examined to what extent the comorbidity was due to genetic and environmental factors that predispose to both alcohol use and smoking. The genetic analyses showed that the underlying factors that influence alcohol and tobacco use and cause their association were different for adolescent and young adult twins. Initiation of alcohol use and smoking in adolescents (aged 12-16 years) was substantially influenced by the same shared environmental features. Alcohol and tobacco use in young adults were associated due to the same genetic risk factors.

A religious upbringing reduces the influence of genetic factors on disinhibition: evidence for interaction between genotype and environment on personality.

Information on personality, on anxiety and depression and on several aspects of religion was collected in 1974 Dutch families consisting of adolescent and young adult twins and their parents. Analyses of these data showed that differences between individuals in religious upbringing, in religious affiliation and in participation in church activities are not influenced by genetic factors. The familial resemblance for different aspects of religion is high, but can be explained entirely by environmental influences common to family members. Shared genes do not contribute to familial resemblances in religion. The absence of genetic influences on variation in several dimensions of religion is in contrast to findings of genetic influences on a large number of other traits that were studied in these twin families. Differences in religious background are associated with differences in personality, especially in Sensation Seeking. Subjects with a religious upbringing, who are currently religious and who engage in church activities score lower on the scales of the Sensation Seeking Questionnaire. The most pronounced effect is on the Disinhibition scale. The resemblances between twins for the Disinhibition scale differ according to their religious upbringing. Receiving a religious upbringing seems to reduce the influence of genetic factors on Disinhibition, especially in males.

The influence of religion on alcohol use initiation: evidence for genotype X environment interaction.

We examined the possible role of religious upbringing as a mediator of the shared environmental influences and as a moderator of the genetic influences on the risk of alcohol use initiation in a large population-based sample of Dutch adolescent and young adult twins (1967 twin pairs). There was not a significant association between religious participation and alcohol use initiation among Dutch adolescents and young adults. We also hypothesized that the relative magnitude of the genetic influences on the risk of alcohol use initiation would be greater for those adolescents and young adults who were raised in a less religious environment compared to those adolescents and young adults who were raised in a more religious environment. We indeed found higher heritabilities for females without a religious upbringing compared to females with a religious upbringing. Genetic influences accounted for 40% of the variance in alcohol use initiation in nonreligious females, compared to 0% in religiously raised females. Shared environmental influences accounted for 54% of the variance for nonreligious females and 88% of the variance in religious females. For males, the genetic variance was also higher in the nonreligious group compared to the religious group, but this difference was not statistically significant. Whether or not they were raised religiously, the liability to alcohol use initiation in males was moderately influenced by genetic factors (30%) and substantially influenced by shared environmental factors (60%).

The genetics of smoking initiation and quantity smoked in Dutch adolescent and young adult twins.

Not much is known about the genetic and environmental determinants of various aspects of substance use in adolescents. This study examined whether the inheritance of initiation of tobacco use in adolescents is independent of the inheritance of the number of cigarettes smoked. Alternative multifactorial threshold models were applied to data on tobacco use in 1676 Dutch adolescent twin pairs. The three models that were considered are (i) the single liability dimension model, (ii) the independent liability dimension model, and (iii) the combined model (CM). The results showed that there is not one underlying continuum of liability to smoking. The CM was the best-fitting model. This model postulates that there are separate initiation and quantity dimensions but allows for the possibility that there are some individuals who are so low on the liability to level of consumption that they are not using tobacco. There were no differences between males and females in the magnitude of the genetic and environmental influences on individual differences in smoking initiation and quantity smoked. Smoking initiation was influenced by genetic factors (39%) and shared environmental influences (54%). Once smoking is initiated genetic factors determine to a large extent (86%) the quantity that is smoked.

Netherlands twin family study of anxious depression (NETSAD).

In a longitudinal study of Dutch adolescent and young adult twins, their parents and their siblings, questionnaire data were collected on depression, anxiety and correlated personality traits, such as neuroticism. Data were collected by mailed surveys in 1991, 1993, 1995 and 1997. A total of 13,717 individuals from 3344 families were included in the study. To localise quantitative trait loci (QTLs) involved in anxiety and depression, the survey data were used to select the most informative families for a genome-wide search. For each individual a genetic factor score was computed, based on a genetic multivariate analysis of anxiety, depression, neuroticism and somatic anxiety. A family was selected if at least two siblings (or DZ twins) had extreme factor scores. Both discordant (high-low) and concordant (high-high and low-low) pairs were included in the selected sample. Once an extreme sibling pair was selected, all family members (parents and additional siblings of the selected pair) who had at least once returned a questionnaire booklet were asked to provide a DNA sample. In total, 2724 individuals from 563 families (1007 parents and 1717 offspring) were approached and 1975 individuals from 479 families (643 patients and 1332 offspring) complied by returning a buccal swab for DNA isolation. All offspring from selected families were asked to participate in a psychiatric interview and in a 24-hour ambulatory assessment of cardiovascular parameters and cortisol. The interview consisted of the WHO-Composite International Diagnostic Interview and was administered to 1253 offspring. In this paper we describe the genetic-epidemiological analyses of the survey data on anxiety, somatic anxiety, neuroticism and depression. We detail how these data were used to select families for the QTL study and discuss strategies that may help elucidate the molecular pathways leading from genes to anxious depression.