Young adult siblings and twins are less alike in cognitive ability in highly educated families than in less educated families

Does sibling and twin similarity in cognitive ability differ by parents’ education? Tina Baier. ZfF – Journal of Family Research, 1-2019, pp. 58-82. https://doi.org/10.3224/zff.v31i1.04

Abstract: Stratification scholars predominantly investigate how differences among children from different families emerge and tend to neglect differences among children from the same family. I study sibling similarity in cognitive ability and examine whether their similarity varies by parents’ education. Although economic approaches and their extensions argue that disadvantaged parents reinforce differences while advantaged parents compensate for differences, I argue that parents may also make equal investments and thus accept differences among their children. I refer to the literature on stratified parenting that demonstrates that parents are engaged differently in child-rearing and their children’s skill formation processes. Because advantaged parents foster children’s talents more individually compared with disadvantaged parents, I propose that sibling similarity is lower in advantaged than in disadvantaged families. Previous studies based on sibling correlations provide conflicting evidence. To account for observable and unobservable differences among siblings, I extend the established sibling correlation approach and study dizygotic and monozygotic twins in addition to siblings. The analyses draw on novel data from a population register-based study of twin families. I find that young adult siblings and twins are less alike in cognitive ability in highly educated families than in less educated families. Hence, my results support the hypothesis concerning equal investments and indicate that stratified parenting has a long-lasting influence on children’s cognitive ability.

Key words: intergenerational transmission; educational inequality; cognitive ability; sibling correlations; twins; Germany

5. Discussion and conclusion

I studied sibling similarity in cognitive ability and asked whether the degree of similarity

varies with parents’ education. In contrast to previous research, I extended the established

sibling correlation approach to DZ and MZ twins. This acknowledges the increasing evidence that genetic variation matters for cognitive ability and allows us to capture shared

family influences more comprehensively, and thus to test rigorously the link between sibling similarity and parents’ education.

To explain a varying degree of similarity, I first referred to economic approaches that

model parents’ investment decisions within the household (Becker/Tomes 1976; Behrman/Pollak/Taubman 1982). Against this backdrop, I tested the hypothesis that sibling

similarity in disadvantaged families is lower for efficiency reasons, whereas highly educated families compensate for, and thus equalize, differences among siblings (Conley

2004, 2008). I then introduced the idea that parents might also invest equally in and accept differences among their children. I drew on the literature on stratified parenting (e.g.,

Cheadle/Amato 2011; Kalil/Ryan/Corey 2012; Lareau 2011; Lareau/Weininger 2003) and

put it in a within-family perspective. Because advantaged parents adopt an active role in

shaping the developmental processes of their children and tend to provide more skillenhancing and specific inputs in line with children’s potentials and needs, I hypothesized

alternatively that siblings from advantaged families are less similar in terms of cognitive

ability compared with siblings from disadvantaged families. 

My analyses yielded two findings. First, young adult siblings, DZ twins, and MZ

twins in highly educated families are less alike in terms of cognitive ability compared

with young adult siblings, DZ twins, and MZ twins in less educated families. This contradicts the hypothesis concerning stratified investments rationales, according to which sibling similarity increases with parents’ social background (H1), and supports the hypothesis concerning equal investments and stratified parenting (H2).

Systematic differences in the degree of similarity in cognitive ability are significant in

the sibling sample. This is in line with US findings for literacy skills (Conley/Pfeiffer/

Velez 2007) but differs from the finding for Germany (Grätz 2018). One explanation of

the divergent findings could be that the families I studied have more children (twins and

at least one sibling) than the families in the study by Grätz (2018). Unfortunately, this

study does not provide information about the variance components in absolute terms. The

ICC is a standardized measure that does not change if the variances of shared family and

child-specific influences in absolute terms change at the same time. Thus, there might be

some variation in the relative importance of shared family influences that did not show up

in the ICC. To evaluate to what extent results differ substantially, we would also need information on the family level variation in absolute terms.

For both DZ twins and MZ twins, the results reveal the same pattern. The similarity

decreases according to parents’ education, though it is not statistically significant. Nonetheless, both the results for the variance components in absolute terms and for the ICC

confirm that shared family influences decrease the more educated parents are. Thus, the

more resources parents have, the more important are processes within the family that accentuate differences within the family.

In addition, I found that the mean level of cognitive ability increases with parents’ education, whereas the relative importance of shared family influences decreases. These divergent

trends show that the same shared family influences that make siblings and twins more alike

are also associated with lower levels of cognitive ability. This is a very important aspect, and

more research is needed to understand what kind of influences affect siblings equally and

hamper the realization of cognitive ability in less educated families. In advantaged families,

by contrast, parents often provide additional inputs that foster children’s talents. These influences are more child specific, which leads to higher levels of cognitive ability and promotes

differences in cognitive ability among their children. Given that differences between siblings

and twins from advantaged and disadvantaged backgrounds remain even as the children grow

older, my results indicate a long-lasting impact of parenting on cognitive ability.

Second, my results show that the association between parents’ educational background and sibling and twin similarity is not affected by the closeness of the sibling and

twin relationship. I thereby address a major limitation of studies on sibling similarity. In a

similar vein, my results reveal a very similar trend for siblings, DZ twins, and MZ twins,

which shows that there is no “twinning effect” – that is, that twins behave profoundly differently from (full) siblings.

However, it is important to note that I used an indicator that was measured at the

same time as cognitive ability. Since the quality of the sibling and twin relationship might

change over the life course, it is important to back up my results – ideally, with longitudinal data. To the extent that there are no profound changes in the sibling and twin relationship until early adulthood, my results are reliable. 

This study is the first to adopt a genetically sensitive approach to sibling similarity in

cognitive ability. The results provide strong indications for parent’s investment decisions

that are not in line with economic theories, rather parents invest equally in their children

but in distinct ways that differ according to parents’ educational background. My findings

challenge the implicit assumption that shared family influences such as parents’ education

influence children in similar fashion. Moreover, if children are raised in advantaged families, shared family influences – those that differ between families – are less important.

Genetically sensitive research can help us to better understand what kinds of parental investment – net of genetic influences – result in within-family stratification, and to formulate informative policy suggestions to enhance the achievements of children from less educated families.


Also: The Social Stratification of Environmental and Genetic Influences on Education: New Evidence Using a Register-Based Twin Sample. Tina Baier, Volker Lang. Sociological Science, February 20, 2019. 10.15195/v6.a6

The relative importance of genes and shared environmental influences on stratification outcomes has recently received much attention in the literature. We focus on education and the gene-environmental interplay. Specifically, we investigate whether—as proposed by the Scarr-Rowe hypothesis—genetic influences are more important in advantaged families. We argue that the social stratification of family environments affects children’s chances to actualize their genetic potential. We hypothesize that advantaged families provide more child-specific inputs, which enhance genetic expression, whereas the rearing environments of children in disadvantaged families are less adapted to children’s individual abilities, leading to a suppression of genetic potential. We test this relationship in Germany, which represents an interesting case due to its highly selective schooling system characterized by early tracking and the broad coverage of part-time schools. We use novel data from the TwinLife panel, a population-register–based sample of twins and their families. Results of ACE-variance decompositions support the Scarr-Rowe hypothesis: Shared environmental influences on education matter only in disadvantaged families, whereas genetic influences are more important in advantaged families. Our findings support the growing literature on the importance of the gene-environmental interplay and emphasize the role of the family environment as a trigger of differential genetic expression.