Psychology says parents who believe every child needs a sibling may be right about one important thing

For some parents, the decision to have another child is not only about wanting a bigger family. There is often another powerful thought behind it: “My child needs a sibling to grow up properly.”

Parents may imagine their children growing up together, sharing secrets, fighting over toys, learning to compromise and eventually becoming lifelong companions. The belief is so common that having a sibling is sometimes treated almost like a developmental necessity. But what does psychology actually say?

Research suggests that siblings can provide important opportunities for social and emotional development, but simply having a brother or sister does not guarantee better development. The relationship children build with each other, and the environment created by their parents, may matter considerably more.

Psychology Says Siblings Can Become A Child’s First Social Laboratory

Psychologist Lev Vygotsky's sociocultural theory emphasizes that children's development occurs through social interaction. From this perspective, another child in the household can create countless opportunities to negotiate, communicate, imitate, cooperate and solve problems.

A toddler who wants the same toy as an older sibling, for example, has to deal with a miniature social conflict. They may learn to wait, negotiate, express frustration or find another solution.


Unlike an adult, a sibling is often operating at a similar developmental level. That can make everyday interactions unusually intense, and potentially instructive.

Research reviews describe sibling relationships as an important developmental context in which children can acquire social and emotional skills, self-regulation and understanding of other people's emotions.

Sibling Relationships Can Teach Children Emotional Regulation

This connects with the psychological concept of emotion regulation, the ability to monitor and manage emotional responses.

Imagine two siblings arguing over a video game. One becomes angry, the other refuses to give up the controller, and eventually they have to find a way to continue living together despite the disagreement.

Repeated interactions like these can provide opportunities to practice perspective-taking, emotional control and conflict resolution. But there is an important catch. Sibling conflict is not automatically beneficial.

A major meta-analysis involving 34 studies and more than 12,000 children and adolescents found that greater sibling warmth was associated with fewer internalizing and externalizing problems, while greater sibling conflict and differential treatment were associated with more problems.

In other words, the psychological benefit is not simply “having a sibling.” It is having a reasonably healthy sibling relationship.

Parents May Want A Sibling Because They Fear Loneliness

There is also a powerful emotional component to this parenting belief. Some parents imagine their child growing up alone and worry about loneliness, social isolation or having nobody close to share childhood experiences with. A sibling can therefore become psychologically associated with belongingness, companionship and social security.

This connects with Attachment Theory, developed by psychologist John Bowlby, which emphasizes the importance of close emotional relationships in human development.

However, attachment should not be oversimplified into the claim that a sibling automatically becomes an attachment figure or guarantees emotional security. The quality and consistency of relationships matter.

A sibling can become an important source of comfort and companionship, but siblings can also compete, reject one another or remain emotionally distant.

Psychology Says Siblings Can Model Behaviour

Children also learn by observing other children. This idea connects with Albert Bandura's Social Learning Theory, which proposes that people can acquire behaviors by observing and modeling others.

An older sibling who reads frequently, participates in sports, helps around the house or communicates confidently can become a behavioral model for a younger child.

For example, a five-year-old may become interested in books partly because they repeatedly see an older sibling reading. The younger child is not literally inheriting the older child's intelligence; they are being exposed to behaviors that may become familiar and desirable.

The reverse can happen too. Children can model disruptive, aggressive or unhealthy behaviors. That is why research describes sibling relationships as capable of functioning as both risk and resilience factors.

Having More Siblings Does Not Automatically Mean Better Development

This is where popular parenting wisdom becomes more complicated.

A 2023 longitudinal study using three decades of data examined how changes in sibship size related to cognitive and sociobehavioral development. The researchers did not find a simple “more siblings equals better development” pattern; in fact, they reported net decreases in cognitive test scores as family size increased, with effects varying according to children's birth position.


That finding is important because adding another child can also change the family environment.

Parents have to divide time, attention and resources among more children. Older children may gain opportunities for independence, but they may also experience jealousy or reduced parental attention.

This is closely related to the psychological concept of resource dilution, in which family resources can become distributed among a larger number of children.

The Quality Of The Relationship Matters More Than The Number

Modern research increasingly points toward a more nuanced conclusion. A 2024 evidence map examining 277 empirical studies on sibling relationships identified dimensions including warmth, affection, cohesion, positive engagement, hostility, power, conflict and conflict management. Researchers therefore study sibling relationship quality rather than treating “having a sibling” as a simple yes-or-no developmental advantage.

This means parents should be careful about telling an only child, “You need a brother or sister to become social.”

Children can develop friendships, empathy, communication skills and emotional resilience through relationships with parents, extended family, teachers and peers.

Research on birth order also challenges many popular assumptions. A large study using data from more than 20,000 participants found a small birth-order association with intelligence but no lasting birth-order effects on major Big Five personality traits such as extraversion, emotional stability, agreeableness and conscientiousness.

So psychology does not support the idea that every child needs a sibling to become emotionally healthy, socially capable or successful.

What psychology does suggest is more interesting: a supportive sibling relationship can become a powerful developmental environment. The goal, therefore, is not simply to give a child a brother or sister.

It is to create a family environment in which children have opportunities to connect, cooperate, disagree safely, repair conflicts, learn from one another and feel equally valued.

FAQs

Does every child need a sibling for healthy development?

No. Research does not establish that having a sibling is a developmental requirement. Children can develop strong social and emotional skills through relationships with parents, relatives, friends, teachers and other peers.

Can having a sibling make a child more social?

It can provide additional opportunities to practice communication, cooperation, negotiation and conflict management. However, the effect depends heavily on the quality of the sibling relationship.