How Siblings Profoundly Shape Each Other’s Identities

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A recent cover story in The New York Times Magazine by Susan Dominus, the author of the new book, The Family Dynamic, explored the underappreciated but powerful and long-lasting influence siblings exert on each other.

Parents, she points out, have been identified as the most profound forces in shaping children. The influence of siblings on each other has not received nearly enough attention.

Eighty percent of American children grow up with at least one brother or sister. Yet, for more than a century, psychological research has largely ignored the importance of sibling relationships. Even Freud refers to the sibling relationship only five times in his two dozen volumes of work. Only during the past two decades or so have researchers conducted meaningful studies on how siblings shape one another’s lives.

In childhood, brothers and sisters are our first playmates and our first rivals. From them, regardless of age difference or position in the family, we first begin to learn the necessary social qualities—tolerance, generosity, loyalty—that eventually affect relationships with friends, colleagues, and lovers.

Siblings typically spend more time together than with anyone else. For the fortunate—both in lifespan and in the quality of the relationship— sibling relationships may continue for many decades, outlasting most friendships, marriages, and relationships with parents. Siblings who are close in age may find it impossible to remember life without each other.

The common ground of siblings includes childhood experiences and family history, but it starts with genetic makeup. Interestingly, siblings are genetically more closely linked to each other than they are to either parent or anyone else in the world, sharing about 50 percent of genetic material.

In Dominus' article, portraits of several families highlight how these crucial sibling relationships shape identity, influence behavior, and even affect mental and physical health.

Here are some of Dominus’s conclusions:

Co-Parenting Dynamics

An older sibling sometimes becomes a caregiver to a younger brother or sister. This can benefit both siblings, as the dynamic fosters a sense of responsibility and leadership in the older sibling, while providing a sense of security and support for the younger one. Research conducted at the University of Cambridge found that sibling co-parenting can nurture the younger sibling's emotional development, fostering higher levels of empathy and social competence. This dynamic can provide a blueprint for future social interactions and relationships outside the family.

Emotional Complexities

Two emotional imperatives, opposite yet both powerful, exert pressure on sisters and brothers. One is differentiation. Siblings are hard-wired to distinguish themselves from each other, the better to “win” the competition for a limited, crucial resource necessary to survival: parental care. This rivalry can be fierce.

A second, conflicting impulse is the urge siblings feel to invest deeply in relationships with relatives, thereby creating bonds of deep affection and loyalty. One evolutionary theory, kin selection, suggests that the more closely individuals are related, the more likely they are to help one another. And, of course, there’s no closer relative than a sibling.

Dominus suggests that siblings teach each other specific skills: how to handle conflict, how to negotiate, how to empathize. Studies have found that sibling rivalry can promote resilience and problem-solving skills, as siblings learn to navigate conflicts and find resolutions.

What Sibling Research Reveals

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Dominus cites research indicating that siblings with close bonds fare better in life. These studies show:

• Adolescents who perceived that their siblings validated their beliefs and feelings reported higher levels of self‐esteem.
• Sibling support and a strong sibling relationship are correlated with better academic performance.
• For children experiencing or at risk for various damaging situations—poverty, family discord, divorce, parental mental illness—the steady presence of an emotionally stable person, possibly an older sibling, improved their chances of becoming a well‐adjusted adult.
• Sibling support and closeness were associated with reduced levels of loneliness and depression, as well as greater satisfaction later in life.

Support During a Crisis

Finally, Dominus spotlights the value of a supportive sibling during a crisis. A loving brother or sister can provide a unique form of understanding and solidarity, mitigating stress during an upheaval. This enduring connection can be a lifeline.

A strong sibling connection may serve as the cornerstone of emotional health: The longest study of well-being—the Harvard Study of Adult Development, underway since 1938—has found that a close relationship with a sibling during college years provided a reliable indicator of emotional health in later years.

“Siblings, at their best, can urge one another on," Domius writes, “Competing and collaborating—whether intentionally or not—they help chart the course of one another’s lives.”