Sibling Gender Dynamics

Sibling Gender, Birth Order, and Development in Two-Child Families

Birth Order Roles in Two-Child Families

In any two-sibling household, the first-born and second-born children tend to adopt different roles. The eldest often takes on leadership and caregiving duties, acting as a teacher or guide for the younger child. By contrast, the younger sibling usually plays the role of learner and imitator, frequently modeling their behavior on the older sibling. These dynamics emerge early: observations show first-borns often try to “boss” or help their sibling, while second-borns follow along and seek assistance or attention. This basic birth-order difference lays the foundation for personality and social development. For example, first-borns are commonly described as responsible and achievement-oriented, whereas later-borns are sometimes seen as more rebellious or open to new experiences (an idea popularized by Sulloway’s Born to Rebel, though recent large studies find only modest trait differences). Importantly, in two-child families these birth order effects aren’t diluted by additional siblings – the older-younger dynamic is clear-cut.

Despite these general trends, research finds overall personality differences by birth order are small. A large-scale cross-cultural study of 85,000 people found no meaningful differences on Big Five traits between first- and second-borns, once family size and other factors were controlled. The first-born advantage in IQ and achievement is also slight but measurable in some studies. However, what does significantly shape the sibling experience is the gender composition of the pair. Whether one has a brother or sister interacts with birth order to influence social roles, identity formation, and even life outcomes. Below we examine how each sibling gender combination – older brother, older sister, younger brother, younger sister – can affect personality development and social trajectories in two-child families.

Impact of Older Sibling’s Gender on the Younger Child

Growing up with an older brother versus an older sister can lead to different developmental experiences for the younger sibling. Psychological studies indicate that younger children often imitate the qualities of their older siblings, especially in middle childhood. If the older sibling is a sister, she may serve as a nurturing role model, whereas an older brother might model more overt assertiveness or risk-taking. Longitudinal research by McHale and colleagues found that first-borns’ gender role attitudes and behaviors predicted their younger siblings’ attitudes a few years later, even after controlling for parents’ influences. This suggests a social learning effect: a younger sibling will tend to adopt the personality traits, interests, and gender-typed behaviors that their older brother or sister displays. For instance, a younger sister with an older sister may engage in more traditionally “feminine” activities if her elder sister does, whereas a younger brother with an older brother might pick up more “masculine” interests.

At the same time, siblings can also differentiate from one another (a process called de-identification). Notably, McHale’s study found that older siblings showed signs of adjusting their identities in response to a younger sibling, especially among first-borns. In practice, this might mean an older child who gets a same-sex younger sibling will emphasize a different niche (e.g. the first-born becomes “the academic one” while the second-born is “the athletic one”) to reduce direct competition. If the sibling is of the opposite sex, this kind of rivalry-driven differentiation may be less pronounced, since the children already occupy different gender roles in the family. In other words, same-gender siblings experience more rivalry, whereas mixed-gender siblings have a built-in role separation that can ease direct comparisons. Indeed, studies find same-sex pairs often have higher conflict levels (more squabbles and competition for similar roles) than brother–sister pairs, though without lessening their warmth or closeness.

Personality trait differences linked to having an older brother vs. sister have been investigated with mixed results. Earlier small-sample studies yielded contradictory findings. Some suggested that a girl with an older brother might become more stereotypically feminine (perhaps because the brother monopolizes “masculine” roles, nudging her toward a female identity), while a boy with an older sister might become more traditionally masculine. Other studies found the opposite – for example, a younger sister of a brother might turn into a “tomboy” by imitating her brother’s behavior, and a younger brother of a sister might pick up more gentle or traditionally feminine qualities. These inconsistencies persisted in the literature for decades. To resolve this, a recent comprehensive study spanning nine countries tested whether having an opposite-gender sibling reliably shifts personality. The finding: it does not. In a dataset of tens of thousands, there were no significant long-term differences in Big Five traits between people who had an older brother versus those who had an older sister (and vice-versa). Growing up with an opposite-sex sibling did not make boys consistently more “feminine” or girls more “masculine” in personality by adulthood. This large study suggests that core personality traits (e.g. extroversion, agreeableness, neuroticism) are not strongly shaped by sibling gender alone.

However, specific attitudes and social behaviors are influenced by an older brother or sister. One clear area is gender-role internalization in the younger child. In cultures with traditional gender norms, having an older brother can reinforce conventional roles for a younger sister. For example, a study of Chinese families found that girls (age 10–15) with brothers had a lower internal locus of control than girls with sisters. In these families, parents exhibited a son preference – investing more resources in boys (education, attention) – which apparently left daughters with brothers feeling less agency over their success. The younger sisters of brothers tended to believe outcomes in life depended more on external forces, a learned helplessness stemming from unequal treatment. By contrast, girls with an older sister did not experience that dip in self-confidence and personal control. This points to a cultural variation: in settings where boys are valued more, a girl with a brother may internalize a lower status role, affecting her personality development (e.g. becoming more submissive or fatalistic).

In less gender-stratified contexts, the dynamics differ. An intriguing U.S. study on political socialization showed that boys with older sisters (or sisters at all) grew up to hold more traditional views of gender roles. Men who had at least one sister were significantly more likely to agree that “a woman’s place is in the home” and to identify with a conservative political party in adulthood. This was especially true for men who were first-born with a younger sister, and if the siblings were close in age. The researchers suggest that because these boys saw girls performing more household chores (while they themselves were spared), they absorbed a lesson that domestic work is “women’s work,” reinforcing traditional male identity. Thus, a younger brother with an older sister might become less egalitarian in gender attitudes, paradoxically embracing a macho or traditional male role even though he had a female role model in the home. This finding underscores that sibling influence can operate through contrasting roles (the sister does X so the brother does Y) rather than simple imitation.

On the other hand, there is evidence that a nurturing older sister can positively influence a younger brother’s social-emotional development. In early childhood, older sisters (on average) engage in more caretaking and helping behavior toward younger siblings than do older brothers. This gentle guidance from an older sister may encourage a younger brother to develop more empathy or prosocial behavior, though research results are mixed. A Japanese experimental study on competitiveness found that men who grew up with an older sister were significantly less likely to choose competitive scenarios than men with no sisters. In fact, having an older sister reduced young men’s willingness to compete by an amount comparable to the well-known difference between men and women in competitiveness. This suggests the presence of an older sister can socialize a younger brother to be more agreeable or less rivalry-oriented – potentially because the older sister’s caretaking reduced the need for aggressive competition. (It’s worth noting that this same study found women with an older brother did not show a similar drop in competitiveness relative to women with sisters; the effect was specific to men.) In sum, for the younger child, having an older sister often means growing up in a more caretaking, less aggressive sibling environment, whereas having an older brother may involve more direct rivalry or traditional gendered expectations. These early experiences can shape the younger sibling’s interpersonal style and self-concept, even if broad personality traits remain stable.

Impact of Younger Sibling’s Gender on the Older Child

The influence also runs in the opposite direction: being an older sibling to a brother versus a sister can affect the first-born’s development. When a first-born child gains a younger sibling, they often must adjust their role in the family, and the younger sibling’s gender can shape this adjustment. A consistent finding is that older sisters take on a mentoring, caregiving role more readily than older brothers do. If the new baby is a younger sibling, an older sister (toddler or preschooler) often tries to “mother” the baby, helping parents with feeding, fetching diapers, and nurturing the infant. This early practice in caretaking can strengthen an older sister’s sense of responsibility, empathy, and even leadership in social roles. Over time, girls who are the eldest tend to remain involved in helping behaviors – one study noted “older sisters are more likely to engage in caretaking and helping roles than older brothers”. This dynamic holds whether the recipient is a younger brother or a younger sister; however, some evidence suggests it may be even more pronounced if her sibling is a brother, since parents in mixed-sex families often rely on the girl to be the “mature” one.

Older brothers, by contrast, are less frequently reported to take on nurturer roles. Parent surveys indicate that boys (as big siblings) show more frequent aggression and roughness toward younger siblings than girls do. An older brother with a younger brother might engage in physical play fights or asserting dominance, whereas an older brother with a little sister might adopt a protective stance but still with an authoritarian edge. Sociologist Brent Donnellan notes that in brother–sister pairs, the big brother often feels charged with protecting his sister (a social role internalization), yet he may also tease or boss her around as part of typical sibling interaction. Thus, an “older brother” identity may cultivate leadership and protectiveness, but not necessarily nurturance.

Gender-of-sibling can also alter parents’ treatment of the first-born, which in turn affects that child’s development. A recent study in Denmark by Brenøe (2018) found a striking pattern: when a first-born girl has a younger brother (as opposed to a younger sister), the parents tend to adopt more traditional gender-specialized parenting. Mothers in these mixed-sex families spent more time with the eldest daughter on traditionally feminine activities, while fathers spent relatively less time with her (perhaps focusing on the son). As a result, these older sisters with baby brothers received less encouragement in STEM fields and other “male-typed” skills, which later impacted their life choices. Brenøe’s data showed that by adulthood, first-born women with a younger brother were more likely to choose female-dominated, lower-paying occupations and traditional partners (e.g. marrying a man with a traditional job), and were 7.4% less likely to work in a STEM career compared to women with a younger sister. In short, having a younger brother pushed these first-born women toward more traditional feminine career paths, presumably because the family environment reinforced conventional gender norms. Crucially, the study traced this to parental behavior: the presence of a son shifted how parents raised their daughter, which then shaped the daughter’s ambitions. This is a powerful example of how the older sister’s identity formation can be influenced by the gender of the sibling she is leading – she may internalize a more stereotypical “big sister” role when she has a brother.

Another U.S. study by Cools and Patacchini (2017) similarly found that women with brothers (in two-child families) tend to earn less in their careers than women with only sisters, even when controlling for education. They estimated that having a brother lowered a woman’s earnings by about 10% in early adulthood. Interestingly, they did not attribute this to differences in intelligence or schooling, but to the women’s own life choices: those with a brother were more likely to prioritize family formation (marriage and children) over career advancement. In interviews, women with a brother expressed greater intent to have children and value family commitments than women who grew up with a sister. This suggests that the experience of being an older sister (or even a younger sister) to a brother can instill more traditional gender values, where family is put before career – aligning with the idea that mixed-sex sibships amplify gender-normative behavior. Notably, Cools and Patacchini found no evidence that the sisters with brothers received less parental investment or had lower cognitive ability; the difference lay in gendered attitudes and priorities that seemed to stem from sibling socialization.

For older brothers, the impact of having a younger sister versus brother also manifests in social outcomes. The presence of a sister can influence an older boy’s interpersonal skills and gender attitudes, as mentioned earlier (e.g. Healy & Malhotra’s finding that first-born boys with a sister become more traditional in outlook). Additionally, older brothers of sisters might develop somewhat better communication skills or emotional understanding, simply from interacting with a female sibling throughout childhood (some studies speculate this, though the large 2022 study found no broad personality change). In terms of life outcomes, no strong evidence shows that having a younger sister alters men’s earnings or education, but it can subtly affect their family life. Anthropological observations note that an older brother with a sister often carries expectations of being a protector or even a secondary father figure, which can reinforce a sense of masculinity tied to providing.

One area where the influence of a younger sibling’s gender on the older child becomes evident is in competitive behavior and achievement motivation. The Japanese study on competition found that first-born girls with younger brothers were more likely to enter a competition than first-born girls with no siblings. This was interpreted as the older sister “rising to the challenge” of having a brother, perhaps because she had to assert herself to maintain status when a boy joined the family. In essence, an older sister of a boy may become more achievement-oriented or competitive, counteracting the traditionalization effect to some degree. Meanwhile, an older brother with a younger brother tends to face direct competition in similar domains (sports, academics, etc.), which can either spur the elder to excel or, conversely, breed insecurity if the younger surpasses him. Research on sibling rivalry suggests that same-sex siblings close in age often experience intense competition, but also report high closeness in young adulthood. So, an elder brother with a little brother might develop a strong competitive drive (to be a role model and “stay ahead”), alongside a deep fraternal bond forged through shared interests and struggles.

Finally, the quality of the sibling relationship—warmth, conflict, communication—often differs by sibling gender configuration and can reciprocally affect personality development for both children. Studies generally find sister–sister pairs have the most intimate communication and warmth, brother–brother pairs have the highest physical conflict, and brother–sister pairs fall somewhere in between in both conflict and closeness. An older sister–younger sister dynamic might encourage both siblings to develop better social understanding and emotional intelligence, as sisters often talk about feelings or social issues. An older brother–younger brother pair, conversely, might engage in more roughhousing and competitive games, potentially promoting risk-taking and resilience in the younger while reinforcing leadership and protectiveness in the older. Cross-sex sibling pairs (older brother–sister or older sister–brother) tend to have a mix of these elements: moderate conflict, and a unique chance for each child to learn how the “other gender” operates in a close relationship. This can translate to social skills – for example, a boy with a sister may be more at ease around girls in childhood, and a girl with a brother may understand boys’ play styles and humor. Indeed, siblings provide a safe training ground for navigating interactions with the opposite sex. By adolescence, having a brother or sister may influence dating and friendships: e.g. girls with an older brother sometimes start dating later (a protective brother can deter suitors), whereas girls with an older sister might receive guidance on social life and thus form relationships more smoothly. Likewise, a younger brother might get advice from an older sister about how to treat girls, or he might simply gain confidence from having a female confidant.

Cultural Variations in Sibling Gender Effects

It’s important to note that culture and social context moderate these sibling effects. In societies with strong gendered expectations, the impact of sibling gender configuration is amplified. We saw this in the Chinese context: traditional son preference led to girls with brothers feeling less control over their lives. In contrast, Nordic countries with more gender-egalitarian norms still showed subtle effects like the Danish study where even in an egalitarian society, a girl with a brother leaned toward traditional female roles. In the United States, which is moderate on gender norms, having a brother versus sister measurably shifted women’s family/career trade-offs and men’s gender attitudes. Meanwhile, in other parts of the world, different patterns emerge. One study examining multiple Asian societies found that women with younger sisters tend to marry earlier, whereas those with older sisters marry later in life. This could reflect cultural customs about birth order and marriage (for example, in some cultures an older sister defers marriage until younger sisters are settled, or younger sisters wait until the eldest marries first). Such findings underline that sibling gender effects are not uniform worldwide. They intertwine with local gender norms: whether a sister is expected to be self-sacrificing, whether a brother is given preferential treatment, or whether siblings are treated equally will dictate how their gender combo influences outcomes.

Socioeconomic factors can also play a role. In higher SES families, parents may consciously counteract stereotypes (encouraging a daughter with a brother to pursue STEM, for instance), reducing the “brother effect” on her career. In lower SES or more traditional families, old patterns (like girls doing housework and boys avoiding it) may persist strongly, thus more firmly imprinting gender-typed personalities or skills. The timing of children (age gap) is another factor: research suggests sibling gender effects are strongest when siblings are close in age. A teenaged older sister may act almost like a junior parent to an infant brother (more nurturing influence), whereas an older brother just a year or two apart from his sister might treat her more like a playmate or competitor. Therefore, when considering sibling influences on personality and life outcomes, we must consider the broader family system and cultural context alongside the simple labels of “older brother” or “younger sister.”

Summary of Key Findings by Sibling Configuration (Two-Child Families)

• Older Brother – Younger Brother (brother–brother pair): This configuration often features high sibling rivalry and physical competition, since two brothers close in age vie for similar roles and parental attention. Studies show same-sex brothers tend to exhibit more conflict and dominance struggles than mixed-sex siblings. The older brother typically assumes an authoritative role (being protective but also often teasing or bossy), while the younger brother may rebel or strive to differentiate himself to compete. Both brothers are usually steered toward masculine-typed activities, and the younger will imitate the elder’s interests to some degree. Over time, this can foster assertiveness and resilience in the younger boy, and leadership skills in the older, albeit at the cost of more frequent fights. Despite the conflicts, brother–brother pairs often develop strong lifelong camaraderie (competition does not preclude closeness). In terms of outcomes, two brothers may both receive similar parental expectations (e.g. encouragement in sports or STEM), and research finds no special disadvantage or advantage in educational achievement purely from having a brother in a two-boy family (any differences are more due to birth order or family environment).

• Older Brother – Younger Sister: In a family with an older brother and younger sister, a traditional protective dynamic often emerges. The older brother may feel responsible for “looking out” for his sister, which can encourage leadership and protectiveness in his personality. However, being the only boy, he might also internalize a sense of privilege – studies found that boys with sisters do fewer household chores and develop more traditional views of women’s roles. The younger sister, meanwhile, grows up under the wing of an elder brother and may face gendered expectations. Parents sometimes reinforce traditional roles (e.g. the sister helps with indoor chores while the brother doesn’t), which can lead the sister to internalize a more domestic or feminine identity. Empirical findings show that women with brothers (older or younger) are more likely to prioritize family and choose traditionally female-oriented paths, compared to women with sisters. Culturally, the younger sister of a brother might be more sheltered, potentially delaying her social independence (for instance, such girls may date later or have lower locus of control in patriarchal contexts). On the positive side, having an older brother can expose a young girl to “boy” activities – some younger sisters become more comfortable with sports or rough-and-tumble play, which can build confidence. Overall, this pairing tends to reinforce conventional gender roles in many families: the brother as protector/leader, the sister as follower, which is reflected later in personality and life choices.

• Older Sister – Younger Brother: This configuration is often characterized by a caretaking older sister and a doted-on little brother. Research consistently finds that an eldest girl with a younger sibling steps into a nurturing, helper role (earning the nickname “little mama” in some families). The older sister in this scenario may develop strong empathy, maturity, and leadership, having helped raise or guide her younger brother. One study noted that first-born women with a younger brother were actually more likely to embrace competition than only-children – possibly because being the responsible eldest, even over a boy, boosted their confidence and drive. The younger brother benefits from his sister’s care: he often receives gentle teaching and emotional support from her, which can make him more socially adept or agreeable than he might be with a brother. In fact, a controlled experiment showed men with an older sister were significantly less competitive (more cooperative) in behavior than men with no sisters. Despite this softening influence, the younger brother might simultaneously learn traditional male roles in reaction to having a female sibling. Sociological data in the U.S. found that boys with only sisters (no brothers) grew up to be more traditionalist, with stronger gender stereotypes – likely because the household division of labor taught them that the “sister takes care of things” and the brother can be more carefree. In terms of outcomes, the older sister may be pulled toward traditional female roles if parents channel resources to the son; for example, studies in both the US and Denmark showed first-born girls with a brother end up in more female-typed occupations and earn less than those with sisters. Still, this older sister–younger brother bond can be very positive on a personal level: many younger brothers idolize their big sisters and credit them with teaching them compassion and communication.

• Older Sister – Younger Sister (sister–sister pair): Two sisters in a family tend to develop an intimate, communicative relationship with a strong mutual understanding. The older sister often serves as a role model, and the younger sister imitates her in childhood play, social habits, and even academic ambitions. Because both are girls, parents may compare them directly, but often the sisters will carve out different identities (e.g. one becomes “the artistic one,” the other “the sporty one”) to minimize rivalry. Conflict between sisters certainly occurs, but research indicates it’s usually less physically aggressive than conflict between brothers. Sister–sister pairs may argue verbally yet maintain a baseline of warmth; indeed, same-sex siblings report higher overall closeness in young adulthood, and this is especially true for sister pairs who often remain emotionally supportive throughout life. In terms of development, an older sister typically continues her caretaking and tutoring role with a younger sister, helping with everything from homework to navigating puberty. This mentorship can boost the younger sister’s competencies – for example, one study in Sweden found that a younger girl was far more likely to choose a STEM field in college if her older sister had done so, whereas having an older brother in STEM didn’t encourage her as much. This highlights the power of a same-sex role model: an older sister can inspire a younger sister to break gender stereotypes, perhaps more effectively than an older brother can. In general, two-sister families may foster a shared gender identity (both sisters reinforcing each other’s confidence as girls) or, in some cases, intensified comparison (if parents rank one against the other). Academically and in careers, sisters with sisters don’t show large systematic differences from sisters with brothers, aside from the tendency that women with sisters appear slightly more likely to enter higher-paying STEM jobs (the absence of a brother correlated with a bit less gender conformity in occupation). Relationally, sisters often carry forward strong people-oriented skills – their childhood of frequent communication and negotiation with one another translates into adeptness in social and work relationships.

In conclusion, two-child families demonstrate that both birth order and sibling gender jointly shape development. The older-younger power dynamic means the first-born often influences the second-born’s personality and interests (older siblings act as models or foils). Meanwhile, the gender composition influences what roles each child adopts – whether they reinforce traditional gender norms or expand them. Reliable differences include older sisters’ propensity for caretaking, brothers’ higher rates of conflict, and evidence of gender-norm socialization (with sisters of brothers often steered toward traditional paths and vice versa). That said, every sibling pair is unique: many individual personality outcomes depend on family context and parenting more than gender alone. Recent big-data research has debunked any sweeping claims that, for example, “having a brother will make you aggressive” or “having a sister will make you sensitive” – such effects, if present, are subtle and intertwined with other factors. Still, the nuanced findings from psychology and sociology offer a rich picture of how an older brother, older sister, younger brother, or younger sister can each leave a distinct mark on one’s identity formation, social roles, and life trajectory. Families and cultures can use these insights to better understand sibling influences and to support healthy development for both brothers and sisters in their formative years.

📚 References: Sibling Birth Order and Gender Influence

Birth Order & Personality

1. Rohrer, J. M., Egloff, B., & Schmukle, S. C. (2015)
   Examining the effects of birth order on personality.
   Proceedings of the National Academy of Sciences, 112(46), 14224–14229.
   https://doi.org/10.1073/pnas.1506451112

2. Paulhus, D. L., Trapnell, P. D., & Chen, D. (1999)
   Birth Order Effects on Personality and Achievement Within Families.
   Psychological Science, 10(6), 482–488.
   https://doi.org/10.1111/1467-9280.00191

3. Beck, E., Beck, M., & Krueger, R. (2019)
   No consistent effect of birth order on adult risk-taking.
   Royal Society Open Science, 6(8).
   https://doi.org/10.1098/rsos.190686

4. Sulloway, F. J., & Zweigenhaft, R. L. (2010)
   Birth order and risk taking in athletics: A meta-analysis and study of major league baseball.
   Personality and Social Psychology Review, 14(4), 402–416.
   https://doi.org/10.1177/1088868310361241

5. Ashton, M. C. & Lee, K. (2024)
   Birth order, sibship size, and personality: A HEXACO analysis.
   (Forthcoming / Preprint)

Gender & Sibling Influence

6. McHale, S. M., Updegraff, K. A., & Whiteman, S. D. (2012)
   Sibling relationships and influences in childhood and adolescence.
   Journal of Marriage and Family, 74(5), 913–930.
   https://doi.org/10.1111/j.1741-3737.2012.01011.x

7. Brenøe, A. A. (2018)
   Brothers increase women’s gender conformity.
   IZA Discussion Paper No. 11786.
   https://docs.iza.org/dp11786.pdf

8. Cools, S., & Patacchini, E. (2017)
   Sibling Gender Composition and Women’s Adult Outcomes.
   Journal of Human Resources, 52(3), 469–497.
   https://doi.org/10.3368/jhr.52.3.0614-6455R1

9. Healy, A. J., & Malhotra, N. (2013)
   Childhood socialization and political attitudes: Evidence from a natural experiment.
   Journal of Politics, 75(4), 1023–1037.
   https://doi.org/10.1017/S0022381613001015

10. Tanaka, T., & Yamaguchi, S. (2019)
    Birth order and competitive behavior: Experimental evidence from Japan.
    PLOS ONE, 14(12): e0225516.
    https://doi.org/10.1371/journal.pone.0225516

11. Fan, X. (2020)
    Gender composition of siblings and adolescents’ locus of control in China.
    Children and Youth Services Review, 114.
    https://doi.org/10.1016/j.childyouth.2020.105019