Once you've taken account of the quality of sibling

When Robert J. Waldinger declared: “Once you’ve taken account of the quality of sibling relationships, knowing about the quality of parenting doesn’t add much information,” he unveiled a truth that many overlook: that the bond between siblings can rival, and even surpass, the influence of parenting itself. In this observation lies the recognition that parents are not the only shapers of a child’s destiny. Brothers and sisters, bound by blood and circumstance, forge one another in the fires of rivalry, loyalty, and companionship, often in ways that parents cannot.

The origin of this insight comes from Waldinger’s lifelong work as a psychiatrist and director of the Harvard Study of Adult Development, one of the longest studies of human happiness and flourishing ever conducted. The study revealed that while parenting lays the foundation, the quality of sibling relationships often shapes how that foundation is lived out. For it is with siblings that children spend countless hours, navigating conflict, sharing secrets, learning to compete, and learning to reconcile. Siblings become the first mirrors in which one sees oneself reflected, distorted, challenged, and understood.

History provides vivid examples. Consider Cleopatra of Egypt and her siblings, who together shaped the fate of empires. Her survival and rise to power were not simply the result of her father’s influence but the fierce and often deadly rivalries within her own family. Or look to the Wright brothers, Orville and Wilbur, whose collaboration as siblings birthed the age of flight. Their achievement was not merely the product of their parents’ upbringing, but of the deep trust and unity they cultivated together as brothers. In both ruin and triumph, sibling bonds proved decisive.

Waldinger’s words remind us that while parents may guide, it is often siblings who test, refine, and accompany one another into adulthood. The shared experiences of childhood — meals, quarrels, adventures, hardships — weave threads that continue to bind long after the parents’ guiding hands have faded. Where parenting creates structure, siblings create practice. Where parenting imparts values, siblings test them in the crucible of daily living. Thus, the quality of sibling relationships becomes a predictor of resilience, empathy, and happiness.

The emotional truth of this quote is profound. A loving sibling can be a lifelong ally, a refuge in storms, a reminder that one is not alone in the world. Conversely, a fractured sibling bond can leave wounds that no amount of parental love can heal. Parents may strive to nurture, but if siblings live in bitterness and hostility, the house itself becomes divided. Waldinger’s insight urges us to recognize the sacred responsibility not only of raising children, but of nurturing the bonds between them.

The lesson, then, is this: do not underestimate the power of siblings in shaping the destiny of a family. Parents must not only pour their wisdom into their children individually but must foster love and respect between them. For children who learn to cherish one another carry that strength into the wider world, while those who grow in rivalry and resentment often carry hidden burdens into adulthood. To raise a family is not only to raise children, but to raise their relationships with each other.

Practical actions must follow: parents, encourage cooperation more than competition among your children. Teach them to resolve conflicts with fairness, and to celebrate each other’s victories without envy. Create rituals that bind siblings together — shared meals, shared responsibilities, shared stories. And siblings themselves, as you grow, remember that your bond is a lifelong inheritance. Guard it, honor it, and heal it when it breaks, for it is one of the greatest treasures you possess.

So let Robert J. Waldinger’s words be carried forward as ancient wisdom: parenting sets the stage, but sibling bonds write the play. In the laughter, quarrels, and reconciliations of brothers and sisters lies a power that can determine the course of a life. Parents will one day pass, but siblings remain, and the quality of those relationships will echo through the years as a source of strength or sorrow. Choose, then, to nurture those bonds, for in them lies the true measure of family.