The research question explores the impact of sibling gender composition on sex differences in cognitive abilities and occupational choices. Social learning theories suggest that same-sex siblings might reinforce gender-typical behaviors. The study investigates whether individuals with same-sex siblings at age 16 differ in academic strengths (verbal/language vs. technical/numerical) compared to those with opposite-sex siblings. Furthermore, it examines if this effect persists in occupational choices at age 35. A substantial body of literature documents sex differences in academic strengths, with females excelling in verbal tasks and males in spatial and numerical tasks. These differences often mirror academic performance and occupational choices, with women overrepresented in verbally demanding fields and men in numerically demanding ones. Previous research on the role of sibling gender on gender-typical behavior has yielded mixed results. Some studies suggest that same-sex siblings lead to more gender-typical behavior, while others find no significant effect. There is a lack of research specifically examining the influence of early sibling composition on cognitive strengths and career choices. One study indicated that a higher proportion of boys in a sibship increases the likelihood of pursuing STEM fields. Another showed that girls with brothers tend to choose more female-dominated occupations. This study uses high-quality Swedish register data from the 1973-1982 birth cohorts to examine the impact of sibling composition on academic strengths at age 16 and occupational choices at age 35, controlling for relevant covariates.

Existing literature demonstrates consistent sex differences in academic abilities, with females generally performing better in verbal tasks and males in spatial and numerical tasks. These differences are reflected in academic performance and occupational choices, resulting in gender segregation in various fields. Social learning theories propose that same-sex siblings may reinforce sex-typical behaviors. However, studies on the influence of sibling gender composition on gender-typical behavior have yielded mixed results, with some showing stronger gender-typical behavior in same-sex sibling groups and others showing no significant effects. Prior research on educational and occupational choices has shown varying results; some studies suggest that having more male siblings correlates with increased likelihood of pursuing STEM fields, while others indicate girls with brothers may opt for female-dominated professions. This study addresses the gap in research examining the impact of early sibling gender composition on cognitive strengths and career choices, using a large, representative dataset.

Two studies were conducted using data from Swedish national registers covering the 1973-1982 birth cohorts. Study 1 focused on the middle child in 3-sibling families (N=17,233), comparing those with same-sex (GGG, BBB) and opposite-sex siblings (BGB, GBG). Study 2 analyzed the youngest child in 2-sibling families (N=118,688), comparing same-sex (GG, BB) and opposite-sex sibling compositions (BG, GB). Academic strengths at age 16 were assessed using normalized 9th-grade school grades in Swedish, English, Math, and Technics. A composite score was calculated by subtracting the sum of verbal subject grades (Swedish and English) from the sum of numerical subject grades (Math and Technics). Positive scores indicate verbal strength, negative scores numerical strength. Cognitive demands of chosen occupations at age 35 were assessed using a coding scheme based on the Swedish Standard Classification of Occupations (SSYK). Occupations were rated on verbal and numerical task frequency, and a composite score was created (subtracting verbal from numerical demand). Multiple linear regression models were employed to analyze the relationship between sibship composition and academic strengths/occupational demands, controlling for GPA, mother's age and education, sibship type (identical parents or not), ethnicity, and sibling age difference. A 2x3 factorial ANOVA examined the interaction between academic strengths and sibship composition on occupational demands. Sensitivity analyses were conducted to address potential confounding factors like parental identity and sibling age spacing.

Both studies revealed a significant main effect of sex on academic strengths at age 16, with girls showing more verbal alignment and boys more numerical alignment. However, sibship composition did not significantly affect academic strengths in either study. Study 2 showed a significant association between sibship composition and boys' occupational choices, with boys having an older sister tending towards more numerically demanding occupations compared to those with an older brother (p=0.022). This effect was small (Cohen's d=0.014). Moderator analyses in Study 1 showed no significant interaction between sibship composition and academic strengths on occupational choice. In Study 2, a significant interaction was found (p=0.029), indicating that boys with verbally aligned academic strengths were less likely to choose numerically demanding occupations if they had an older brother. This effect was more pronounced in boys with verbally aligned academic strengths at age 16 (p = 0.005). Sensitivity analyses, considering only siblings with identical parents and analyzing spacing effects, confirmed the main findings.

The findings challenge the hypothesis that growing up in a gender-typed environment significantly alters individual academic strengths or occupational choices. While expected sex differences in academic strengths were observed, these strengths proved resistant to the influence of sibling gender composition. The observed trend among boys in Study 2, indicating a potential link between having an older sister and selecting numerically demanding occupations, requires further investigation, especially given the small effect size. This suggests that factors beyond sibling gender play a dominant role in shaping occupational pathways. The study's findings suggest that individual cognitive abilities may be more resistant to environmental influences than occupational choices. The resilience of academic strengths to environmental factors aligns with research showing the stability and heritability of cognitive performance. The susceptibility of occupational choices to environmental and societal factors highlights the complexity of career development.

This study demonstrates that having same- or opposite-sex siblings does not significantly influence the verbal or numerical alignment of academic strengths, which exhibit robust sex differences. Although a small trend was observed, indicating that boys with older sisters might choose numerically demanding occupations, this was not consistent across both studies. The lack of a significant effect suggests that the influence of sibling gender composition on career paths is limited, particularly for females. Future research should investigate the underlying mechanisms contributing to the observed trend among boys and explore other environmental factors influencing occupational choices.

The study relies on school grades as a proxy for academic strengths, potentially influenced by non-cognitive factors or teacher bias. While the grades used were based on standardized tests, this remains a limitation. Another potential confound concerns the comparability of skill levels across different occupations in the verbal-numerical demands comparison. Further, the study focuses on Swedish data, limiting its generalizability to other cultural contexts.