Sibling behavior and relationships grow out of a blend of the genes children inherit, the relationships around them at home, and the experiences each child has on their own. The surprising part is that growing up in the same house does far less to make siblings alike than most people assume.1PubMed Central. Why are children in the same family so different from one another? How brothers and sisters treat each other is shaped partly by their parents, partly by their ages and genders, and partly by the siblings themselves.
Why Children in the Same Family Turn Out So Different
Two adopted children raised in the same family from early in life end up barely more alike in personality than unrelated strangers.2Behavioral and Brain Sciences. Why are children in the same family so different from one another? Most of the environment’s effect on personality, mood problems and thinking skills comes from experiences each child has on their own.3PubMed Central. Why are children in the same family so different from one another? Siblings can end up as different from each other as two children picked at random.4PubMed Central. Why are children in the same family so different from one another?
What counts as a separate experience in the same house? Different friends, teachers and activities give each child a different world outside home. A parent at 25 and the same parent at 35 are effectively different parents to the first and second child. A family move or a divorce hits a toddler and a teenager very differently. Each child’s temperament also changes how adults respond to them. Children also shape their parents’ behavior: an easygoing baby draws different responses than an intense one, so the same parents end up in a different relationship with each child.
That is why blaming or crediting parenting for every difference is a common mistake. Genes give each child a different starting point even with identical care.
Genes also shape how siblings act toward each other. Identical twins behave toward each other more similarly than non-identical twins do, so genes play a modest to moderate part in how warm or difficult a child is with a brother or sister.5PubMed Central. Using Twins to Better Understand Sibling Relationships
How Parents’ Relationship Spills Into Sibling Fights
Hostility between parents and harsh parent-child relationships are linked to more sibling conflict and even sibling violence.6PubMed Central. Sibling Relationships and Influences in Childhood and Adolescence In teenagers, more conflict between parents goes with more hostility between siblings, while a warm, close couple goes with more sibling affection.7PubMed Central. An integrated model: marital effect on adolescent behavioral problems through siblings Sibling hostility, in turn, is tied to more aggressive behavior and low mood in teens.8PubMed Central. An integrated model: marital effect on adolescent behavioral problems through siblings Children may copy the arguing they see, or feel pushed to take sides, which strains their bond with each other.9PubMed Central. An integrated model: marital effect on adolescent behavioral problems through siblings
The route runs through children’s own feelings. Kids notice tension even when parents think they are hiding it, and unsettled children tend to carry that stress into the people closest to them.
Parents can argue away from the children when possible and let them see the making up. Never ask a child to side against the other parent.
These links show that tension at home and sibling conflict tend to appear together, not that parental fights are the only cause of sibling fights. Temperament, age and stress outside the home also feed sibling conflict, so a squabbling pair doesn’t mean the parents’ marriage is failing. In one study of Black families, a spike in parental conflict hurt sibling warmth only when money worries were lower than usual.10Family Relations. Context matters: Longitudinal associations between marital relationships and sibling relationships in Black families The pattern is a tendency, not a rule for every household.
Favoritism and Treating Children Differently
Treating siblings differently is moderately linked to more acting-out behavior and to how much affection siblings feel for each other.11Journal of Social and Personal Relationships. Parental differential treatment, child’s externalizing behavior and sibling relationships: Bridging links with child’s perception of favoritism and personality, and parents’ self-efficacy What most predicts hostility toward a sibling, though, is a child’s sense of being less favored.12Journal of Social and Personal Relationships. Parental differential treatment, child’s externalizing behavior and sibling relationships: Bridging links with child’s perception of favoritism and personality, and parents’ self-efficacy Favoritism, meaning unequal treatment a child reads as preference, can last. Even in young adulthood, getting less support from a father than a sibling does is linked to more depressive symptoms, and the effect was strongest in same-gender pairs when fathers treated them very differently.13PubMed Central. “Life Still Isn’t Fair”: Parental Differential Treatment of Young Adult Siblings
Two children in the same home can disagree about whether treatment is fair, and the one who feels shortchanged is more likely to act out against a sibling. Parents may not notice because unequal treatment often follows a natural affinity, such as sharing a hobby or temperament with one child. Favored children can be affected too, since they may feel pressure to keep their standing or guilt about a sibling’s hurt.
Some different treatment is perfectly fair. Children of different ages and needs reasonably get different rules and help. Explaining why treatment differs helps a child see it as fair rather than as unequal treatment. Children compare attention, warmth and time more than gifts, and asking each child privately whether things feel fair to them shows what they actually see. To reduce the sense of favoritism, give each child regular one-on-one time, avoid comparing children aloud, and name the reason when one child gets something the other doesn’t.
Birth Order, Age Gap and Gender
Earlier-born children score slightly higher on intelligence tests on average, and later-borns rate their own intellect a little lower, by about a tenth of the typical spread between people.14PubMed Central. Examining the effects of birth order on personality Firstborns also tend to perceive more sibling conflict than middle children do.15The Family Journal. Siblinghood, Gender, and Families
The bossy firstborn and rebel youngest are stereotypes with little support. Measured effects are small averages hiding big individual variation, and family size and spacing often explain more than position. A difference of a tenth of the typical spread is far too small to predict anything about one particular child. And because many families expect a firstborn to be responsible, children may act as expected, so some stereotypes keep themselves going.
Firstborns may feel more conflict because they go from being the only child to sharing their parents’ attention, a change later children never experience. Older children are also often asked to give way or take responsibility, which can feel unfair and make conflict more noticeable.
Spacing and gender change how strongly siblings affect each other. The quality of a sibling relationship is more strongly tied to children’s emotional and behavior problems when siblings are close in age.16Elsevier / PubMed Central. Sibling relationship quality and psychopathology of children and adolescents: a meta-analysis The tie is also stronger in pairs with more brothers, and in childhood than in the teen years.17Elsevier / PubMed Central. Sibling relationship quality and psychopathology of children and adolescents: a meta-analysis In one group studied, sisters reported more conflict and brothers more closeness.18The Family Journal. Siblinghood, Gender, and Families Close-in-age siblings share friends, toys and attention, so friction and influence tend to be frequent.
Sharing Parents’ Time and Finding Separate Niches
Parents’ time, energy and money are limited, so each added child can mean less for each one.19PubMed. Number of siblings and intellectual development. The resource dilution explanation. How much this matters depends on the setting, such as gender preferences and inheritance customs.20The History of the Family. Siblings and life transitions: investigating the resource dilution hypothesis across historical contexts and outcomes Families use their resources in very different ways, so a large family can still give each child strong attention. Older children in large families sometimes give younger ones care and teaching, which adds to what parents provide.
When resources are thin, attention matters more than money for closeness. Short focused moments can stand in for long outings, and rotating special roles, like choosing the movie or helping with dinner, spreads attention around.
Siblings also ease rivalry by becoming different from each other. Those who carve out clearly different interests and identities tend to compete less.21PubMed Central. The Nature and Correlates of Sibling Influence in Two-Parent African American Families Siblings who are already naturally different may not need to differentiate to keep competition low.22PubMed Central. The Nature and Correlates of Sibling Influence in Two-Parent African American Families
Niche-picking, such as one child becoming the athlete and the other the artist, can box a child in. Letting each child try the other’s interests without comparison keeps the doors open.
What Siblings Teach Each Other
Toddlers who cooperate more with an older sibling get better at understanding others’ thoughts and feelings within months.23PubMed Central. A Reciprocal and Dynamic Development Model for the Effects of Siblings on Children’s Theory of Mind Warmth between young siblings predicts more empathy in the older child a year and a half later.24PubMed Central. A Reciprocal and Dynamic Development Model for the Effects of Siblings on Children’s Theory of Mind In one study, autistic children with an older sibling did as well as typical children on a test of understanding others’ beliefs.25PubMed. Theory of mind in children with autism spectrum disorder: do siblings matter?
Pretend play with a sibling requires each child to imagine what the other is thinking, such as playing along with a made-up story. Older siblings often explain things, offer comfort and negotiate, giving younger children practice they might not get alone.
Conflict can teach or harm. Sibling fights can teach children to find win-win solutions, manage emotions and see the other side.26Encyclopedia on Early Childhood Development. Sibling Relations and Their Impact on Children’s Development But early antagonism toward a new baby predicted weaker understanding of others only when mothers responded with little child-centered discipline.27PubMed Central. Theory-of-Mind Development and Early Sibling Relationships after the Birth of a Sibling: Parental Discipline Matters With a new baby, child-centered discipline means talking about the baby’s feelings and the older child’s feelings, explaining rather than only punishing, and praising gentle moments.
Coaching works better than refereeing. Ask each child to say what they want and how the other feels, and help them suggest solutions instead of imposing one. Step in firmly for hitting or bullying.
Special Family Situations
Siblings of a brother or sister with a disability or chronic illness often feel invisible, guilty or weighed down by family roles.28Journal of Community & Applied Social Psychology. Glass children: The lived experiences of siblings of people with a disability or chronic illness Support from family and others in the same situation helps them feel understood.29Journal of Community & Applied Social Psychology. Glass children: The lived experiences of siblings of people with a disability or chronic illness Many adults who grew up this way report personal growth and stronger people skills, alongside worries about embarrassment and future relationships.30PubMed. Growing up alongside a sibling with a disability: A phenomenological examination of growth and deficiency in adulthood Studies disagree on whether their overall quality of life is lower, and parents rate it lower than children do.31PubMed. Impact of growing up with a sibling with a neurodevelopmental disorder on the quality of life of an unaffected sibling: a scoping review
Parents’ attention often goes to the child with greater needs, so the other child may stop asking for help to avoid adding to the strain. They may feel guilty about their own health or complaints, and angry, then guilty about the anger. Adult-like jobs given early, such as watching a sibling, can build maturity but also weigh on them.
It is wrong to assume every such sibling will struggle, and asking the child directly is more reliable than guessing. Scheduling time focused only on them helps, as does inviting their feelings, including resentment, without judgment. Many disability organizations run sibling support groups.
Research on half and step siblings is growing but still mostly compares them with full siblings rather than studying what helps them.32Journal of Family Theory & Review. Half‐Sibling and Stepsibling Relationships: A Systematic Integrative Review Closeness takes time and can’t be forced, the age at joining the family shapes the bond, and fair rules across both households help.
Culture matters too. In many immigrant families, older children are expected to supervise and raise younger ones.33PubMed. Sibling caretaking in immigrant families: understanding cultural practices to inform child welfare practice and evaluation Asian American firstborns describe a cultural duty to help immigrant parents and care for younger siblings.34PubMed Central. Perceptions of Sibling Relationships and Birth Order among Asian American and European American Emerging Adults
How Sibling Relationships Change in Adulthood
Older adult siblings in one study were in touch roughly once or twice a week on average.35PubMed Central. Sibling Relationships in Older Adulthood: Links with Loneliness and Well-being Siblings who call and text more often feel closer, and frequent contact goes with greater life satisfaction.36Journal of Social and Personal Relationships. Adult sibling relationships and media multiplexity theory Regular calls and texts keep siblings up to date, so conversations don’t start from scratch, and a quick message or shared joke shows the other person they are in mind. Sharing online games or media links to warmer feelings between young adult siblings at any distance.37PubMed Central. Sibling relationship quality in the context of digital leisure and geographic distance for college-attending emerging adults
Regular short calls beat rare long visits, and group chats or games keep connection going. Old childhood roles can be renegotiated as adults.
Caring for aging parents is a common source of disputes, especially when one sibling seems to take over a parent’s care and money and cuts the others out of decisions.38PubMed Central. Tensions among siblings in parent care Sharing information and involving everyone early can head off conflict. Family meetings about care and finances help, as does dividing tasks openly by ability and distance and putting decisions in writing.