Participants and measures
Data from the Netherlands
The Netherlands twin register (NTR) is a nation-wide population-based register founded in 1987 in the Netherlands [24]. For the present study, we included mother-reported data for 7-year-old twins (N = 24,826) born between 1986 and 2006. We excluded twin pairs in which one or both twins had a disease or disability that interfered with daily functioning (N = 714). The final sample consisted of 12,056 twin pairs (N = 24,112 twins, M age = 7.45 years, SD = 0.40, 49.7% males). Socioeconomic status (SES) was based on parental level of education. Based on the highest educational qualification of either the mother or the father assessed at age seven, we categorized children’s SES as low, medium, or high.
Behavioral problems were assessed using maternal ratings on the Aggressive Behavior syndrome subscale of the Child Behavior Checklist (CBCL) [1]. This scale consisted of 18 items that assessed aggressive and non-aggressive behaviors such as “Disobedient at home”, “Gets in many fights”, and “Sulks a lot”. Mothers were asked to report on their child’s behavior in the past 6 months. Response categories were: 0 = “Not true”, 1 = “Sometimes or somewhat true”, and 2 = “Very true or often true”. If more than three items were missing, participants were not included; otherwise, the mean score was imputed for missing items.
Data from the United Kingdom
The Twins early development study (TEDS) is a twin register that longitudinally follows the development of twins born between 1994 and 1996 in England and Wales (from here on referred to as the United Kingdom) [25]. For the present study, we included parent-reported data for 7-year-old twins (N = 20,685). We excluded 515 twin pairs in which one or both twins had a disease or disability that interfered with daily functioning. The final sample consisted of 9,822 twin pairs (N = 19,644 twins, M age = 7.07, SD = 0.25, 48.7% males). Like in the NTR, SES was based on parental level of education for the TEDS sample. Based on the highest educational qualification of either the mother or the father, assessed at first contact, we categorized children’s SES as low, medium, or high. Although the educational system differs between the Netherlands and the United Kingdom, we established comparable categories, as displayed in Table 1.
Behavioral problems were assessed using parental (97.3% maternal ratings) ratings on the Conduct Problem subscale of the Strengths and Difficulties Questionnaire (SDQ) [2]. This scale consisted of five items that assessed aggressive and non-aggressive behaviors such as “Often fights with other children or bullies them”, “Generally obedient, usually does what parents request”, and “Often has temper tantrums or hot tempers”. Parents were asked to report on their child’s behavior. Response categories were: 0 = “Not true”, 1 = “Somewhat true”, and 2 = “Certainly true”. If more than two items were missing, participants were not included; otherwise, the mean score was imputed for missing items.
Statistical analysis
To gain insight in the distribution of childhood behavioral problems across sex, SES, and countries, we obtained descriptive statistics using R. Next, we performed twin analyses in R (version 3.4.3) with the package OpenMx (version 2.8.3) with the NPSOL optimizer [26].
Twin analyses
With twin models, by comparing resemblance on a trait between monozygotic (MZ) and dizygotic (DZ) twins, it is possible to disentangle to which extent individual differences in a trait can be explained by genetic variance (A), variance due to the shared environment (C), and variance due to the nonshared environment (E) [12]. We extended the model by including two moderators to test whether the contribution of genetic and environmental variance to individual differences in childhood behavioral problems interacted with these moderators (i.e., SES strata and country).
Because the distribution of childhood behavioral problems was highly skewed, we categorized the variable by applying two thresholds, partitioning the sample in the 33% lowest scoring, the middle 33%, and 33% highest scoring children on childhood behavioral problems. This method has the advantage of optimizing parameter estimates [27]. We fitted the thresholds for the Netherlands and the United Kingdom separately, before entering them into the model. To simultaneously compare SES strata and countries, we fitted a 30 group model containing all groups (e.g., MZ male, DZ male, MZ female, DZ female, DZ opposite sex × SES low, SES medium, SES high × the Netherlands and the United Kingdom). Categorizing SES into low, medium, and high allowed us to test for both protective and negative moderating effects of (high/low) SES. Because studies so far did not find evidence for qualitative or quantitative sex differences for childhood behavioral problems [18, 28], we constrained the correlations and A, C, and E components to be equal for boys and girls and opposite-sex twin correlations to be equal to DZ correlations. To account for the frequently observed mean differences in behavioral problems, thresholds were allowed to vary across sex.
Model fitting
We tested moderator effects by stepwise testing whether constraining parameters to be equal across moderator categories significantly deteriorated goodness of fit (i.e., p < 0.01). If a constraint did not significantly deteriorate model fit, we proceeded with applying this constraint in the later models. Based on the best fitting model, we estimated the influence of genetic and environmental factors on childhood behavioral problems across SES strata and countries.
First, we fitted a saturated model with thresholds freely estimated across sex, SES strata, and countries, and with correlations freely estimated across SES strata and countries. Next, we fitted the following models to test for the moderating effects of SES and country: thresholds constrained to be equal across SES strata; correlations constrained to be equal across SES strata; thresholds constrained to be equal across country; and correlations constrained to be equal across country.
Based on the findings from the saturated model, we specified the ACE model with the same 30 groups, constraining thresholds and A, C, and E in line with the results from the previous models to test the moderating effect of SES and country on the contribution of genetic and environmental factors to childhood behavioral problems. We first compared the ACE model to the best saturated model. Next, we tested the moderator effects by constraining A, C, and E across moderator categories.
For interpretational purposes, we performed additional analyses; we fitted the best ACE model but then with the thresholds constrained and the means and variances freely estimated. This model allowed us to examine the absolute variance of childhood behavioral problems across SES strata and countries. Based on this model, we reported the absolute values of A, C, and E.