This section presents the results on the effect of sibling sex on women’s adult outcomes and their children’s school performance, using the main sample. Section 4.1 provides the main results of the paper by eliciting the effect of sibling sex on women’s gender conformity in terms of their (1) choice of occupation and partner and (2) differential response to motherhood in labor market outcomes. Next, Section 4.2 considers the role of education and family formation as potential channels of the effects on labor market outcomes. Finally, Section 4.3 examines whether the effects persist to the next generation by studying the school performance of the children of the women in the main analysis.
Choice of occupation and partner
Figure 1 illustrates the main results on the impact of having a second-born brother compared to a sister on women’s choice of occupation and partner. Overall, having a brother enhances women’s conformity to traditional gender norms. First-born women with a second-born brother work in occupations with 1.2% fewer men compared to first-born women with a second-born sister. Note that this difference in occupational choice is observed well into these women’s labor market careers during their thirties (as an average from the ages of 31 to 40). Consistent with this, having a brother also reduces women’s probability of working in STEM fields by 0.4 percentage points, corresponding to a decrease of 7.3% relative to the mean for women with a sister. Consequently, the results clearly show that having a brother induces women to exhibit more traditional choices of occupation. In other words, they are less prone to opt into traditionally male-dominated occupations, including STEM.
Sibling sex also has a significant impact on the choice of partner in terms of the degree to which his occupation is gender-typed. Having a brother rather than a sister induces women to choose a partner who works in more male-dominated occupations. On average, women with a brother have a partner working in occupations with 2.0% fewer women than women with a sister.
Table 3 shows the regression-based results, with different control versions. Column (1) replicates the raw mean differences between first-born women with a second-born sister and those with a second-born brother from Fig. 1, while column (2) includes basic demographic controls. Column (3), the preferred model, further controls for parental education. Finally, column (4) flexibly adds controls for family size and the sex of potential third- and fourth-born siblings.Footnote 21 As family size is an outcome of sibling sex composition, the latter control version might bias the estimates. However, this control version works as a robustness check of the results, as family size might also be considered a confounding variable. Regardless of the covariates included, the estimates across the different control versions are almost identical, supporting the assumption that sibling sex is random and illustrating that family size is not a principal mediator of the effect of sibling sex. The rest of this paper proceeds by presenting the results using the preferred control version in column (3).
As a test of the robustness of the main measures of gender conformity, Appendix Table 9 considers two alternative measures. Notably, having a brother also increases the partner’s relative earnings in the couple and the age between the woman and the partner. These results demonstrate a consistent effect of having a brother not only on women’s choice of gender-stereotypical occupations and partners but also on other aspects of their gender-conforming behavior.
If the effect of sibling sex at least partly is attributable to the way in which parents treat their children, we might observe some heterogeneity by parental characteristics in the effect of having a brother.Footnote 22 Panel A in Table 4 includes an interaction term between sibling sex and an indicator for having parents working (almost) equally during childhood. Remarkably, the effect of having a brother on occupational choice disappears for women coming from more gender-equal families. This suggests that women with more gender-stereotypical parents drive the effect of sibling sex on the probability of choosing more female-dominated occupations. Moreover, the results in panel B suggest that the effect of having a brother is strongest for those women with more traditional parents in terms of their educational field. The effects seem to be largest in magnitude for those with a mother who has an academic education within care or administration and for those with a father who has an academic education within STEM.
Furthermore, the effect of having a brother is largest for those with at least one highly educated parent (≥ 12 years of education) for occupational choice. In most cases, a highly educated parent will also imply having a parent with human capital that is traditionally associated with his or her own gender. For instance, most mothers with greater education are in the care and administration fields (e.g., nurse, secretary, and office work) and most fathers are in STEM fields. Therefore, these results again support the previous findings—this time, with an emphasis on parental gender-stereotypical human capital rather than gender-stereotypical labor supply.
Notably, the results also show that women whose parents both have less education do not experience an effect of sibling sex. This suggests that the effect is not due to resource constraints, which has been suggested as a potentially relevant mechanism in the sibling sex composition literature on educational attainment (Amin 2009; Butcher and Case 1994). Cools and Patacchini (2019) also observe this pattern, finding an effect of sibling sex on earnings among women with skilled, but not among those with unskilled, parents. Such finding is further consistent with Charles (2017) who shows that the gender gap in STEM aspirations is larger in more affluent countries. The heterogeneity for the other two outcomes in Table 4 is qualitatively consistent with the findings for the male share in the woman’s occupation, despite being more imprecisely estimated.
Finally, I consider heterogeneity with respect to spacing to the second-born sibling. Spacing might affect child-parent interactions and thereby the impact of sibling sex composition. A first-born daughter with long spacing was the only child for a longer time, during which the parents did not treat her differently based on their next child’s sex. Especially, influences in early childhood and the formation of child-parent interactions might be important for later family dynamics. At the same time, short spacing might increase the (accumulated) exposure to gendered parenting and gender-specific toys in the presence of a brother. Thus, longer spacing might reduce the strength of the impact of sibling sex. Expanding the sample to include women with up to 8 years before their second-born sibling and including interactions between sibling sex and spacing shows, in line with the theoretical predictions, that sibling sex does not have an impact for those with long spacing between them and their sibling (Appendix Fig. 3).Footnote 23 However, the estimated effects by spacing are not statistically significantly different from each other, probably due to the small fraction of children with long spacing between them and their second-born sibling.
In sum, these heterogeneities indicate that the effect of having a brother is strongest for women from more traditional families. In turn, this suggests that differences in child-parent interactions are important for the effects of sibling sex composition on the formation of women’s gender conformity. All other things being equal, we would expect that parents with more gender-stereotypical human capital would transmit gender norms to a stronger extent than those parents with less gender-specific human capital (Humlum et al. 2019). Additionally, we would expect that spending more time with the mother than with the father would influence the child more in the direction of the mother’s (female) rather than the father’s (male) interests. Therefore, the results are consistent with the hypothesis that parents of mixed-sex children invest more time in their same-sex child than parents of same-sex children; Section 5 elaborates more thoroughly on this.
Response to motherhood in labor market outcomes
To shed further light on how sibling sex impacts women’s conformity to traditional gender roles, this subsection examines whether women with a brother respond to motherhood differently than women with a sister in terms of labor supply and earnings. Using data from Denmark similar to mine, Kleven et al. (2019) document that exactly in the year of the first childbirth, female labor supply and earnings experience an immediate drop and never converge back to their initial level, while the arrival of the first child does not affect men’s labor market trajectories. Moreover, Kuziemko et al. (2018) demonstrate that upon motherhood, women in Great Britain adjust their attitudes towards gender roles substantially in a more traditional direction. Based on this evidence, the timing of the first childbirth seems to be a key trigger for women to conform to traditional gender roles. Therefore, studying women’s labor market trajectories by sibling sex before, around, and after their first childbirth might help nuance the picture of the impact of having a brother on the development of gender conformity and improve our understanding of the brother earnings penalty documented in previous work (e.g., Cools and Patacchini 2019; Peter et al.2018).
Graph (a) in Fig. 2 illustrates that in the 6 years preceding the first childbirth, sibling sex does not differentially affect womens labor earnings trajectory once taking out time and individual fixed effects. Remarkably, already in the first year after entry into motherhood, women with a brother experience a larger drop in earnings by 0.33 percentiles relative to women with a sister.Footnote 24 This effect remains stable and statistically significant through 9 years after the first childbirth, i.e., through the end of the period of study. To put this into perspective, women with a younger sister experience a drop in labor earnings by 3.99 percentiles 9 years after their first childbirth, while this number is 4.41 for women with a younger brother. Thus, 9 years after the arrival of the first child, women with a younger brother experience a child earnings penalty that is 10.53% larger than women with a younger sister.
Next, graph (b) explores heterogeneity in the effect of having a brother by childhood family background. For this, I split the effect of sibling sex by parental division of labor during the women’s childhood. Thus, I present the estimates of the effect of having a brother for women of parents working (almost) equally during childhood (referred to as equal in the graphs) and the effect of having a brother for women of fathers working (much) more than mothers (unequal). The negative effect of having a brother on women’s earnings trajectory upon entry into motherhood is entirely driven by women from more traditional families: these women experience a drop in earnings that is 0.5 to 0.7 percentile points larger in the 9 years following their first childbirth compared to the rest of the sample.
Before entering into motherhood, sibling sex does not differentially affect women’s labor supply, measured through their cumulated full-time equivalent work experience (graph (c)). Meanwhile, after the arrival of the first child, a difference by sibling sex emerges. Nine years after entry into motherhood, women with a brother have cumulated 0.54 fewer months of work experience. Again, this difference by sibling sex is solely driven by women from more traditional families: women with a brother from more gender-stereotypical families have cumulated nearly 1 month less of work experience 9 years after the birth of their first child compared to women with a sister (graph (d)). Put differently, women with a brother from more gender-equal families do not experience a differential labor market trajectory upon entry into motherhood relative to the one of women with a sister.
Previous studies on sibling sex composition have documented negative effects of having a brother on women’s earnings (Cools and Patacchini 2019; Gielen et al. 2016; Peter et al. 2018). They have, however, done so without relating the time of measurement to the entry into motherhood and mostly without considering potentially dynamic effects over time. Appendix Fig. 4 illustrates the impact of having a brother on women’s labor market outcomes from an event study, including individual fixed effects, examining whether women experience different labor market trajectories by sibling sex between ages 18 and 40.Footnote 25 This shows that for the overall sample (not restricted to women with at least one child) a negative and statistically significant effect of having a brother on earnings emerges from age 28 and persists through age 40—an effect that is again completely driven by women from traditional families. To relate these results to the ones on the differential response to motherhood, Appendix Fig. 5 displays the cumulative distribution of age at first childbirth. By age 28, 55% of women have had their first child which help explain the timing of the emerging brother earnings penalty.
To compare the magnitude of these results with other studies, Appendix Fig. 4 demonstrates that the negative effect of having a brother on log-earnings in women’s thirties corresponds to a decrease of approximately 2%. Consistent with my results, Peter et al. (2018) find a negative effect of having a brother on a proxy for women’s permanent income in the magnitude of nearly 1% in Sweden. Similarly, Cools and Patacchini (2019) show that first-born women in the USA earn 9% less around age 30 when having a second-born brother instead of a sister.Footnote 26 These similar findings of a negative impact of having a brother on women’s earnings across three different developed countries suggest that my findings on gender conformity might be generalizable to a broader set of countries. At the same time, the differences in magnitudes also suggest that the effects on gender conformity might be larger in more gender unequal societies.Footnote 27 Thus, although the brother earnings penalty is only 2% in my empirical setting, its economic significance seems to be substantial in more gender unequal settings.
Education and family formation
Could differences in ability or fertility behavior explain the impact of having a brother on women’s increased conformity to traditional gender roles in terms of occupational and partner choice? In short, the answer is no. I do not find any evidence of an impact of sibling sex on educational attainment or school performance (columns (2) and (3) in panel A, Table 5).Footnote 28 This is similar to Peter et al. (2018), which is the only existing study with causal estimates of sibling sex on educational attainment. Likewise, Cyron et al. (2017) do not find an effect of sibling sex on girls’ cognitive or non-cognitive skills in first grade in the United States. Thus, sibling sex does not seem to affect differences in ability or (financial constraints in terms of) access to education. This supports an interpretation that the channels of the effect of sibling sex on occupational choice are changes in interests or identity.
While sibling sex does not affect overall educational attainment, the effect of sibling sex on occupational choice is closely mirrored in field of education by age 30. Having a brother reduces the share of men in the highest completed field-by-level of education by 1.35%. Similarly, women with a brother relative to those with a sister are respectively 7.4 and 11.0% less likely to ever enroll in and complete any field-specific STEM education. Appendix Table 10 further shows that the effect is already present in the type of first educational enrollment after compulsory education and that the effect is present for STEM degree completion at different levels of education. Thus, having a brother pushes women out of traditionally male-dominated fields as early as age 16.
The magnitude of the effects is comparable with previous studies examining the impact of various aspects of the social environment in school on study choice (Bottia et al. 2015; Carrell et al. 2010; Schneeweis and Zweimüller 2012; Fischer 2017). Moreover, the results are broadly comparable with other studies examining correlations between sibling sex composition and field of college major (Anelli and Peri 2015; Oguzoglu and Ozbeklik 2016). Appendix Table B13 in Brenøe (2021) displays the associations between the sex of a first-born sibling and second-born women’s gender conformity, indicating similar but less robust correlations compared with the main results. These results are also closer to those in Anelli and Peri (2015), who do not find a significant association for women’s enrollment in high-earning college majors (although the magnitude of their estimate is relatively large). This stresses the importance of rigorously considering selection bias when the aim is to evaluate the causal effect of sibling sex.
A potential reason for the differences in educational and occupational choice by sibling sex could be differences in family formation preferences. Women with a stronger desire to have children early or more children might plan their choice of occupation accordingly, as female-dominated occupations tend to be more family-friendly (Goldin 2014; Kleven et al. 2019). If that were the case, we would expect women with a brother to wish to marry earlier, have children earlier, or have more children than women with a sister.Footnote 29 The administrative data do not report women’s family preferences, but it rigorously documents their actual behavior. Overall, I do not find support of any meaningful impact of sibling sex on the various aspects of family formation reported in panel B in Table 5. The results only suggest a small negative effect of having a brother on cohabitation,Footnote 30 while sibling sex has no effect on the probability of being married (column (2)). Thus, the only difference between women with a brother and those with a sister is that the former move in with a partner before marriage slightly later. This might explain the small positive (though negligible) effect on age at first childbirth.Footnote 31 Overall, sibling sex has no effect on the fertility rate through age 41, i.e., close to complete realized fertility.
Persistent effects to the next generation (of girls)
So far, I have documented that the childhood family environment affects the development of women’s gender conformity. Having a brother influences the family environment to such a degree that women choose more female-dominated occupations and more gender-conforming partners. This motivates the final question—before turning to the study of potential mechanisms—whether the effect on gender conformity (and thereby these women’s adult family environment) is sufficiently strong to affect the next generation. To investigate this, I examine the school performance in Grade 9 in language and math respectively of these women’s first-born daughters and sons separately. In other words, I here focus on school performance in subjects that are associated with traditionally “female” (language) versus “male” (math) skills. In line with the typical finding that boys seem less sensitive to the social environment than girls in terms of “gendered” outcomes (Bottia et al. 2015; Carrell et al. 2010; Fischer 2017), we might expect largest impacts on daughters’ performance relative to the one of sons.
A potential effect of sibling sex on the next generation’s school performance might either go through a direct transmission of gender norms from parents to children or through the type of parents’ human capital. On the one hand, more traditional (gender-conforming) parents might impose more gender-stereotypical expectations on their children than less traditional parents. For instance, traditional parents might not have high expectations for their daughters’ math performance but, in contrast, expect their sons to perform well in math.
On the other hand, parents might have similar expectations but different possibilities to help their children with homework. As mothers are more likely to help children with homework than fathers,Footnote 32 maternal skills might be particularly relevant for this channel. Girls with a more gender-conforming mother might receive more help with language homework and, for instance, be more encouraged to read books for leisure than girls with a less gender-conforming mother. As previously shown (Table 5 and Appendix Table 10), the sex of the mother’s sibling does not affect her own school performance in compulsory education or in overall high school GPA. Yet, sibling sex affects her field of post-compulsory education, changing her competences within certain skill domains. Therefore, girls with a more gender-conforming mother might also receive less(more) qualified help with or be less (more) encouraged to do their math (language) homework. Note, however, that the gender gap in math performance (0.10 SD) is not as large as in language (0.45 SD), suggesting that most of any potential action might happen in the “female” domain of skill acquisition.Footnote 33 Consequently, if having a more gender-stereotypical mother (and father) affects the next generation, we would expect daughters to perform better in languages and/or worse in math.
Remarkably, Table 6 shows that daughters whose mother’s second-born sibling is male relative to female perform 2.44% of a standard deviation better in languages. Meanwhile, I do not detect an effect on daughters’ math performance nor on sons’ performance in either discipline. Thus, daughters’ differences in language and math ability are larger for those with a more gender-conforming mother.Footnote 34 This increase in girls’ absolute advantage in language over math might in turn predict more traditional choices of field of education. Notably, I find evidence of very persistent long-run consequences of women’s childhood family environment. A likely explanation for this finding is the change in daughters’ childhood family environment in terms of the parental skill sets and gender role attitudes, an aspect of the maternal family environment that was unaffected by her sibling’s sex.