Siblings Split: How Unequal Parental Support and Personality Shape Identity

Brothers and sisters grow up under the same roof, with the same parents, the same dinner tables, and often the same rules. Yet ask any family and you will hear how strikingly different the children turned out. One becomes the responsible planner, another the free spirit; one excels at academics, another at music or sport. Developmental psychologists have long been fascinated by this puzzle, and a new study published in Current Psychology by Hamide Gozu and Jenna K. Rieder of Thomas Jefferson University adds a fresh and technically detailed piece to it. The research, which surveyed 517 emerging adults across the United States, suggests that the way parents distribute support between siblings—and the personality of each child—work together to shape how strongly young people try to carve out an identity distinct from their brother or sister.

The phenomenon the researchers investigated is known as sibling differentiation, a term that has largely replaced the older label of deidentification. The idea traces back to Alfred Adler, who argued that when parental resources such as time, attention, and affection seem limited, siblings compete for them, and in response they consciously or unconsciously set themselves apart from one another. By developing different interests, strengths, and roles, siblings reduce direct rivalry and secure their own niche within the family. Decades later, this remains one of the most compelling explanations for why children raised together can be so dissimilar, a question that has occupied behavioral geneticists since the recognition that nonshared environmental influences, not the shared family environment, drive much of the divergence between siblings.

What makes the new study distinctive is its focus on emerging adulthood, the period between roughly 18 and 29 years of age when people are actively constructing their identities. Most previous work on parental differential treatment, the practice of showing affection, granting privileges, or providing support unevenly across children, concentrated on childhood and adolescence. But the authors reasoned that even as young adults gain independence, parents remain a background resource contributing to adjustment and well-being, and that patterns of unequal treatment may continue to influence how siblings position themselves relative to one another. Differentiation during this stage, they note, may help reduce unwanted comparisons from outsiders and ease lingering competition between siblings.

The study also broke new ground by treating mothers and fathers separately. Drawing on developmental theory suggesting that mother-child and father-child relationships play somewhat different emotional and socialization roles, the researchers measured perceived maternal and paternal differential support using an adapted version of the Intergenerational Support Index. Participants rated ten forms of parental assistance, including emotional, practical, communication, advice-related, and financial support, indicating for each whether it flowed more toward themselves or toward a designated sibling closest in age. Scores above the scale midpoint signaled support favoring the participant; scores below signaled support favoring the sibling. Both scales showed strong internal consistency, with reliability coefficients of 0.82 for maternal and 0.83 for paternal support.

Sibling differentiation was measured with the differentiation subscale of the Sibling Influence Scale, which captures the extent to which people deliberately distance themselves from a sibling by emphasizing differences in behaviors, choices, or strengths. A sample item reads, I try to be good at things that my sister or brother isn’t good at. Personality was assessed with Goldberg’s 25-item Adjective Scale, covering the five major traits of openness, conscientiousness, extraversion, agreeableness, and emotional stability. The analytic strategy was ambitious: a single moderated regression model containing covariates such as gender, birth order, age gap, and family size, plus linear and quadratic terms for maternal and paternal support, all five personality traits, and twenty interaction terms testing whether each trait moderated each component of parental support.

The headline finding concerns maternal support. Neither the linear nor the quadratic term of paternal support predicted differentiation on its own, but maternal support showed a significant curvilinear, U-shaped association. Differentiation tendencies were lowest when young adults perceived maternal support as roughly equal between themselves and their sibling, and rose as perceived inequality increased, regardless of whether the imbalance favored the participant or the sibling. This pattern fits equity theory, which holds that perceived inequity in relationships creates psychological tension that motivates behavioral adjustment, and social comparison theory, which predicts that both strongly favorable and strongly unfavorable treatment heighten the salience of interpersonal differences. It also echoes the self-evaluation maintenance model, under which differentiating from a sibling can serve as a strategy for protecting self-esteem when comparisons become threatening.

Personality proved to be a powerful lens through which these dynamics play out, and conscientiousness emerged as the most consequential trait. For maternal support, conscientiousness moderated the curvilinear pattern: participants higher in conscientiousness showed a pronounced U-shape, with the lowest differentiation at equal maternal support and higher differentiation at unequal levels, while those lower in conscientiousness displayed a much flatter profile with only modest variation. The authors suggest that conscientious individuals, who tend to be attentive to expectations, structure, and fairness, may find balanced maternal support particularly reassuring, reducing any perceived need to establish a distinct sibling niche, whereas inequality in either direction reactivates that motivation.

Conscientiousness operated differently for fathers, moderating the linear rather than the curvilinear association. When paternal support favored the sibling, participants lower in conscientiousness showed higher predicted differentiation than their more conscientious peers, but when support favored the participant, the two groups converged. Highly conscientious individuals, who are typically achievement-oriented and well regulated, may manage unfavorable comparisons without resorting to differentiation, whereas less conscientious individuals may experience sibling-favored paternal support as threatening and respond with a reactive, self-protective distancing strategy. Extraversion and agreeableness each moderated the curvilinear link between paternal support and differentiation in opposite ways. More extraverted participants differentiated least when paternal support strongly favored the sibling and most around equal or participant-favored levels, possibly leveraging perceived advantage to assert status. More agreeable participants showed a classic U-shape, differentiating least at equal support and most under inequality, which the authors interpret as a constructive effort to preserve relational harmony by reducing direct comparisons. Less agreeable participants showed the reverse, an inverted U-shape peaking at equal support, perhaps because balanced treatment intensifies competitive comparison for those less concerned with harmony.

The broader message is that maternal and paternal support carry distinct psychological meanings within sibling dynamics. Paternal support may function as a signal of status or validation, encouraging proactive differentiation in some people and defensive differentiation in others depending on dispositional tendencies, while maternal support may shape identity positioning through emotional security and relational comparison, with balanced maternal care creating a stable context that lowers the urgency to differentiate. The findings also carry practical implications: rather than striving to eliminate all differential treatment, which is often unavoidable given children’s differing needs, parents might communicate the rationale behind unequal support, and professionals working with families may benefit from attending to both child personality and parent gender when considering how favoritism is experienced.

The authors are careful to note the study’s limits. It relied exclusively on self-report from one member of each sibling pair, used a correlational design that cannot establish causation, and examined only supportive forms of differential treatment rather than criticism or conflict. The parent-gender findings may not generalize to single-parent or same-gender parent families, and caregiver involvement was not directly assessed. Still, the core conclusion stands out clearly: the processes that push siblings apart do not fade after adolescence. Even in the twenties, as young adults assemble their adult identities, the subtle arithmetic of who gets more parental support—and the personality of the person doing the counting—continues to shape whether they choose to walk a path deliberately different from their sibling’s.

Subject of Research: How parental differential support and personality traits shape sibling differentiation tendencies in emerging adulthood

Article Title: Finding one’s own path: parental differential support, personality, and sibling differentiation tendencies

Article References: Finding one’s own path: parental differential support, personality, and sibling differentiation tendencies. (n.d.). https://doi.org/10.1007/s12144-026-10153-1

Image Credits: AI Generated

DOI: 10.1007/s12144-026-10153-1

Keywords: sibling differentiation, parental differential treatment, personality, emerging adulthood, conscientiousness, extraversion, agreeableness, Big Five, social comparison, equity theory, family dynamics, identity development