In this study we presented an extension of the children-of-twins model, which can be used to study genotype-environment correlation. Because the ECOT model combines data from two different studies the model can be utilized to make maximal use of studies with moderate samples sizes, which are more common. More importantly, the ECOT model allows us to examine three possible mechanisms underlying associations between parent-child relationships and child adjustment: 1) parenting affects the adjustment of children directly through the mechanisms that are independent of the parents or child’s genotype; 2) passive genotype-environment correlation; 3) evocative genotype-environment correlation.
The results of simulations showed that the ECOT model was stable and recovered the true parameter values with high precision. The model was robust to the variance structure and was able to adequately recover both the presence and absence of the evocative and non-evocative paths when passive or evocative rGE was absent or present. It also performed very well when small effects of rGE were present.
When the model was applied to the observed data, the results suggested that the association between maternal emotional overinvolvement and internalizing problems in adolescents was likely to primarily be regulated by evocative rGE. That is, mothers, experiencing their children as anxious, withdrawn or depressed, tended to get more emotionally overinvolved in their parenting. Our findings are consistent with the results of a study by Reitz, Dekovic and Meijer (2006)
where adolescents showing higher levels of internalizing problems were parented with higher involvement, which further increased the problem behavior one-year later. Evocative rGE could also explain the results of Brennan et al. (2003)
, where nondepressed and emotionally overinvolved mothers seemed to have, in contrast, a protective effect for child behavior problems (see above).
Another finding of the same study of Brennan et al. (2003)
was consistent with passive rGE: depressed and emotionally overinvolved mothers were associated with decreased levels of resilient outcomes in youth. That is, problem behavior in children could depend on both inherited genetic liability to depression as well as overprotective parenting. Similarly, passive rGE can explain the results of Hirshfeld et al. (1997)
, where high maternal EOI was associated with separation anxiety disorder only in children of mothers with history of anxiety disorders. Comparing these findings with the ones of our population-based study, one may expect that the type of rGE acting in the association between EOI and child internalizing problems differs depending on mother’s psychopathology. This hypothesis should be tested in a future study, where information on maternal depression and/or other disorders is taken into account.
The direct causality path was nonsignificant and could be excluded from the model. This suggests that maternal EOI does not affect the level of internalizing problems in adolescents in a direct environmental way. However, a study by Reitz et al. (2006)
has shown that parental involvement alone decreased, although insignificantly, internalizing problem behavior measured in adolescents one-year later. The difference in findings between these two studies may depend on different measures of parental involvement as well as to the different designs of the studies.
Finding evidence for evocative rGE has been a common pattern in studies of associations between negative parent-child relationships and antisocial behavior (Burt, McGue, Krueger, & Iacono, 2005
; O’Connor et al., 1998
). According to the results of our study, the same pattern seems to be valid for internalizing problems, too. This is in line with earlier findings in research of development trajectories of child problem behavior. Specifically, a number of studies have suggested that internalizing and externalizing problems often appear and develop together (e.g., Capaldi, 1991
or Gilliom & Shaw).
A study by Lau and Eley (2008)
has examined the co-occurrence of rGE and GxE between maternal punitive discipline and adolescent depressive symptoms. The findings revealed that both rGE and GxE were operating in the development of the association. It is possible that our findings of evocative rGE may also, fully or partly, mask the existence of GxE. The ECOT model alone is unable to test for these processes simultaneously because it employs cross-sectional data. Future longitudinal data studies are therefore needed to further examine this issue.
The main concern when applying the ECOT model is the need to combine two distinct samples. A number of difficulties can arise in attempting to find comparable samples. Even though studies can be matched on a number of characteristics of participants and measurements, differences in, for example, size of correlations still may occur, as was seen in the TOSS and TCHAD samples. Such discrepancies may mainly be the result of different genetic designs of the studies, which does not appear to affect the strength of association between the measures. However, in model application, this may result in larger residuals, leading to a worsened model fit. In our study, large residuals could explain the significant differences between the saturated and full ECOT model. The sample dissimilarity may also have caused the estimates of measurement error to be greater than expected from earlier studies.
Lower intraclass correlations among cousins in TOSS, compared to children in TCHAD, may also depend on the wider age range of the participants. Low correlations among cousins, in comparison to twin children, might result in the underestimated passive rGE. In this study, we have controlled the data for child’s sex and age and therefore believe the possible underestimation of passive rGE, if any, should be negligible.
The ECOT model assumes that passive rGE is present if genes affecting the parental trait contribute to the child adjustment, too. However, it should be noted that because genes might act differently throughout development, it is possible that genes influencing the parental phenotype in adulthood and those important for the child phenotype in childhood are not the same. The effect of passive rGE could therefore be underestimated. One way to deal with this problem would be to include information on child adjustment in parents. However, such extensive developmental studies are difficult to accomplish.
The ECOT as well as general children-of-twins models are estimated by applying cross-sectional data. The conclusions we can draw reflect therefore only a particular time point in the development of parent-child relationships. For the traits with heritability changing over time (e.g., antisocial behavior), the ECOT model can serve only as the first step in explaining the development of the relationships in a family. On the other hand, if no sample size restrictions are present, a cross-sectional children-of-twins model could be extended to a longitudinal model, which would capture the changes over time.
This study has several strengths. The ECOT model can be used to evaluate the type of genotype-environment correlation even when information on only one parent is employed, as is common in many existing twin studies where the children are twins. Also, the ECOT model can be used in studies with commonly used sample sizes. Together with a possibility to combine different study designs, this makes the model particularly useful and relatively easy to apply. The findings on the processes underlying the association between maternal emotional overinvolvement and internalizing problems in children are, to our knowledge, reported for the first time.
However, there are several noteworthy limitations. First, children-of-twins models require large sample sizes for detailed analyses of family relationships. In our study, the data were corrected for the age and sex of the child. Separate analyses on the relationships between mothers and their sons or daughters would shed more light on the potentially different processes involved depending on the sex of the child. Our sample size was not, however, large enough to perform such detailed analyses.
Second, children-of-twins models including only one parent should be applied carefully to study dyadic parental phenotypes (e.g. divorce). A report by Eaves et al. (2006) has demonstrated that the model is not reliable for resolving the direct environmental effect from associations due to genetic effects when studying dyadic parental treatment measures. This is not the case for parental treatments affected only by one of the parents (e.g., depression). Although emotional overinvolvement is not directly defined as a dyadic parental measure (i.e. involving both parents), we cannot exclude that it is not affected by the other parent’s behavior. Therefore, the results of our application study should be replicated by including both parents into analyses.
Third, the ECOT model can only be identified when the measurement error is assumed to be equal for both phenotypes. The model should therefore be applied with caution when measurement errors of the phenotypes differ considerably.
Fourth, although power and sample size analyses for reciprocity model have been performed in previous studies (e.g., Heath et al., 1993
), it is important to test the exact power of the ECOT model. This would let to specify the exact sample size requirements for further application studies. However, because of the extensity of calculations, power analyses are not within the scope of the current report.
Fifth, results of the simulations showed that the ECOT model performs less well when we include parental traits influenced also by shared environment (C1). This makes the model mainly applicable to study parental phenotypes that are not influenced by shared environmental contributions. It is worth noting here that the shared environmental influences on parental phenotypes refer to shared rearing experiences of the twin parents who are now adults and/or to current shared experiences that increase the similarity of the twin parents.
Finally, since the ECOT model excludes the spouse of the parent, assortative mating cannot be taken into account. This should be further tested by employing an extended twin kinship model (Maes et al., 2006
In sum, the current study presents an extended children-of-twins model. This model was designed to advance our understanding of the development of parent-child relationships. More specifically, the model allows a direct test of the presence of genotype-environment correlation, even when only samples of moderate sizes are available. This study also finds that the association between maternal emotional overinvolvement and internalizing problems in children is primarily accounted for by evocative rGE.