Why Siblings Become So Different: Genetics, Environment and the Psychology of Individuality

Same family, different people

At family gatherings, Anna starts conversations with strangers and suggests plans before anyone has finished lunch. Her sister Leah prefers a small circle, a long walk and an afternoon restoring old furniture. Both enjoy their work, care about their family and remember the same kitchen table. Yet their interests and ways of approaching the world seem strikingly different.

These fictional adult siblings illustrate a familiar puzzle. If two people share parents, a household and much of their childhood, why do they not become more alike? The question has helped transform developmental psychology: a family is a shared setting, but growing up within it is never an identical experience.

Research points towards inherited differences, relationships, opportunities and developmental processes that influence one another over time. It also reveals how much remains unexplained. Understanding individuality requires examining both the evidence and the limits of the stories we build around it.

Beyond the nature versus nurture debate

Genetics and environment are often presented as opposing teams. Development does not work that way. Inherited characteristics develop within particular surroundings, while people help shape the experiences available to them. A tendency towards sociability might influence joining a club; repeated participation then creates opportunities to practise social skills.

Behavioural genetics investigates sources of differences between people. Heritability describes the proportion of variation in a measured characteristic associated with genetic differences within a particular population and set of conditions. It does not divide an individual's personality into inherited and environmental ingredients (Visscher et al., 2008).

Consequently, saying that someone is “half nature and half nurture” misrepresents what researchers estimate. A trait can show substantial heritability while remaining responsive to experience. If educational opportunities or the range of environments change, estimates may change too. Statistical contributions to variation are different from a complete explanation of how one person developed.

What siblings actually share

Full biological siblings inherit different combinations of their parents' genetic variants. On average, they share about half of their segregating genetic variation through inheritance, rather than being genetically identical. This does not mean half their entire DNA sequence differs: humans share the overwhelming majority of their DNA. Identical twins are the near-identical inherited-genome comparison used in twin research (Hagenbeek et al., 2023).

Siblings may also share housing, income, cultural traditions, schools and neighbourhoods. These provide important common conditions. However, parents can have different working hours, resources and expectations at different stages of family life. A move happens at different ages for each child. Even attending the same school does not guarantee the same teachers or friendships.

Family membership therefore offers overlapping circumstances, rather than interchangeable childhoods. Nor does an apparently shared event necessarily have the same meaning for both children. One might welcome a move as an adventure while another loses a valued friendship.

It also matters what is being compared. General personality traits summarise patterns across many situations. Playing different instruments, choosing different jobs or behaving differently at home does not necessarily imply a large difference on every broad trait. Two apparently contrasting siblings may share important values or tendencies that attract less attention.

The surprising role of nonshared environments

Plomin and Daniels' (1987) influential review asked why children within the same family differ so much. Its central contribution concerned the environment: much environmental variation in psychological outcomes appeared to make siblings different rather than similar.

“Shared” and “nonshared” have technical meanings here. Shared environmental influence contributes to resemblance between relatives raised together. Nonshared environmental influence contributes to differences after the model accounts for genetic resemblance. The distinction concerns estimated effects, rather than simply whether an experience happened inside or outside the home.

Different friends, teachers, opportunities and relationships are plausible contributors. So are different responses to an event both siblings experienced. However, a list of plausible experiences is not a demonstrated explanation of the statistical findings.

A variance component is not a list of causes

The nonshared component includes measurement error: temporary moods, inconsistent questionnaire responses and other unreliability can make siblings appear less alike. Turkheimer and Waldron's (2000) review emphasised the gap between identifying unexplained environmental variance and identifying particular causal experiences.

Plomin (2024) revisited that gap, proposing that some influences may involve unpredictable developmental processes and highly individual experiences. This is an interpretation of an unresolved problem, not proof that environments are meaningless or that development has no causes. “Unpredictable” means difficult to forecast with available knowledge; it does not mean an experience cannot matter.

A small shared-environment estimate does not show that a household is irrelevant to development. Something everyone in a sample receives may be essential while explaining little variation within that sample. Equally, an influence located in the household can contribute to sibling differences if its effects vary between children. These distinctions prevent statistical labels from turning into judgements about families.

Does parenting affect siblings differently?

Parents may offer different levels of independence, affection, discipline or practical support to different children. Some differences respond to age and need; others involve persistent favouritism or unequal expectations. Equal respect does not require treating a teenager and a young child identically.

A meta-analysis linked less-favoured treatment, and greater differences in parenting between siblings, with more emotional and behavioural difficulties (Jensen & Thomsen, 2024). These associations deserve attention, but they do not establish a simple causal direction. Children's behaviour may influence parental responses, while family pressures or reporting differences may affect both.

Research also examines children's contribution to these relationships. A meta-analysis of children-as-twins studies found evidence consistent with genetically influenced child characteristics evoking different parenting responses (Avinun & Knafo, 2014). Another synthesis highlighted genetic and environmental contributions involving both parents and children (Klahr & Burt, 2014).

For example, a cautious child might seek reassurance, while a more impulsive sibling prompts additional reminders. These illustrative feedback loops do not identify anyone's genes or establish why a particular parent behaves as they do. Small shared-environment estimates for adult personality cannot justify concluding that warmth, fairness or reliable parenting are unimportant.

Birth order: science versus familiar stereotypes

The responsible firstborn, overlooked middle child and adventurous youngest offer memorable explanations. Unfortunately, recognising a stereotype in one family is a weak scientific test: contrasting examples can be overlooked, and siblings' age differences can resemble birth-order effects.

Rohrer et al. (2015) analysed large national panels from Britain, Germany and the United States, including comparisons within families. They found no consistent birth-order differences in extraversion, emotional stability, agreeableness, conscientiousness or imagination. Firstborns did average slightly higher on intelligence tests and self-reported intellect.

Those intellectual findings should be kept separate from personality stereotypes. Educational achievement, intelligence-test performance and conscientiousness are related but different outcomes. A small average difference also leaves substantial overlap between older and younger siblings.

The evidence is not uniformly null. Ashton and Lee (2025) examined two large online volunteer samples using the HEXACO personality framework. They found higher average honesty–humility and agreeableness among people from larger sibling groups, and smaller birth-order differences after accounting for family size.

Their broader measures offer a useful challenge to earlier findings, but self-selection, self-report and unmeasured family characteristics limit causal interpretation. HEXACO agreeableness also differs from its Big Five counterpart. The defensible conclusion is that some modest associations remain under investigation. Birth position does not provide a dependable personality profile or explain why two particular siblings differ.

How genes and experiences influence one another

Correlation: people participate in their environments

Gene–environment correlation describes situations in which genetically influenced characteristics are associated with environmental exposure. A classic developmental account distinguished passive, evocative and active forms (Scarr & McCartney, 1983).

Passive correlation can arise because biological parents provide both inherited variants and surroundings. Musically interested parents might pass on relevant tendencies and keep instruments at home. Evocative correlation concerns responses someone elicits: an enthusiastic participant may receive more invitations. Active correlation concerns selecting settings, such as choosing a quiet reading group or a busy sports club.

A systematic review found genetic contributions to variation in measured environments, including parenting and peer-related measures (Kendler & Baker, 2007). This does not mean friendship or a classroom is “in the genes”. It means environmental exposure can partly reflect characteristics of the people experiencing it.

Related molecular research found that parental genetic variants not transmitted to children nevertheless predicted children's education, supporting indirect environmental pathways called genetic nurture (Kong et al., 2018). This finding concerns education and does not automatically extend to every personality trait.

Interaction: context can change an association

Gene–environment interaction is a different idea: genetic associations may vary across environmental conditions. A meta-analysis found that patterns linking socioeconomic circumstances with genetic influences on cognitive ability differed between US and non-US samples (Tucker-Drob & Bates, 2016). It cautions against assuming a universal pattern; it does not establish which national policies produced the differences.

For an everyday illustration of interaction, imagine two learners who respond differently to highly structured versus exploratory teaching. The relevant question is whether differences associated with inherited variation change across those conditions. Simply observing that one child enjoys a lesson more than another would not demonstrate a genetic interaction: researchers need an appropriate design, reliable measurements and replication.

Neither correlation nor interaction makes inherited tendencies a fixed destination. The musical child still needs access, instruction, practice and an interest in continuing.

Friends, school and life experiences

Anna and Leah might have encountered different friendship groups, subjects and opportunities. These could reinforce existing interests or introduce unexpected ones. A teacher's encouragement could matter greatly to one child while another discovers an absorbing activity elsewhere. Such accounts are plausible developmental pathways, rather than verified explanations of these fictional siblings.

Selection complicates interpretation. In a longitudinal study of emerging friendship networks, personality predicted aspects of friendship selection, including choosing people with similar traits (Selfhout et al., 2010). Similar friends therefore do not, by themselves, demonstrate that friends caused the similarity.

Some research offers stronger evidence for environmental effects. A meta-analysis of quasi-experimental education studies found that additional schooling improved intelligence-test performance (Ritchie & Tucker-Drob, 2018). This supports educational influence on cognitive outcomes, without showing that one inspiring teacher explains a sibling difference in personality.

Later transitions also matter. A systematic review and meta-analysis linked life events with generally small, outcome-specific personality changes (Bühler et al., 2024). Starting work, forming relationships and other transitions can accompany development, although selection and overlapping changes complicate causal explanations. Individual responses need not follow the average pattern.

Why personalities change across the lifespan

Personality stability has several meanings. Rank-order stability asks whether relatively sociable people remain more sociable than others. Mean-level change asks whether a group becomes more or less sociable on average. Everyone could change while largely retaining their relative positions.

Longitudinal meta-analyses show both continuity and development. Earlier work identified average increases in traits including conscientiousness and emotional stability, especially during young adulthood (Roberts et al., 2006). An updated synthesis confirmed substantial stability alongside change, with emotional stability increasing across the lifespan (Bleidorn et al., 2022).

Genetically informative longitudinal research also links both genetic and environmental influences with personality continuity; environmental contributions to stability become more prominent with age (Briley & Tucker-Drob, 2014). Environments can therefore help sustain patterns as well as change them. Childhood matters, but it is not an unalterable blueprint. Anna and Leah can remain recognisably themselves while developing new habits, interests and ways of relating.

What twin and adoption studies can really tell us

Twin studies compare resemblance between identical and non-identical twins. Adoption studies examine resemblance involving relatives who share upbringing without the same genetic relationship, or genetic relationships without being raised together. Longitudinal designs add repeated measurements, helping distinguish temporary variation from enduring development.

Polderman et al. (2015) synthesised fifty years of twin research across many human traits and documented widespread genetic contributions. That broad synthesis is not a single estimate for personality, and neither it nor any heritability estimate determines the cause of an individual's characteristics.

A personality-specific meta-analysis found that estimates differed by research design, with twin studies yielding higher estimates than family and adoption studies (Vukasović & Bratko, 2015). Such differences make it useful to compare methods rather than treat one percentage as definitive.

Assumptions, measurement and representation

The classical twin design assumes, among other things, that environmental similarities relevant to the measured trait do not account for the greater resemblance of identical twins. This is more specific than assuming twins receive identical treatment. Parental resemblance, gene–environment interplay and model specification can also affect estimates. Reviews examine these assumptions and ways to extend the designs (Hagenbeek et al., 2023).

A recent methodological critique highlights difficulties in separating additive from non-additive genetic variance using classical twin models (Hussain, 2026). That is a limitation on interpreting components of variance, rather than evidence that all twin findings are invalid.

Adoption research faces selective placement and restricted ranges of family circumstances. Questionnaires introduce reporting error; samples may underrepresent particular cultures or social conditions. Comparing differently exposed identical twins helps control inherited differences, but remaining confounding still prevents automatic causal conclusions (McAdams et al., 2021).

Molecular methods provide another comparison. A September 2026 genome-wide study found associations involving many personality-related variants and used within-family analyses to examine familial confounding (Schwaba et al., 2026). Its estimates concern measured common variants, rather than the same quantity as total twin-based heritability. No result supplies a genetic forecast of a person's complete identity.

Individuality is more than inheritance or upbringing

Anna and Leah's differences need not reveal a parenting failure, a birth-order destiny or a single decisive event. They have inherited overlapping biological starting points, encountered partly different worlds and participated in shaping their experiences. Some pathways are well supported; others remain difficult to identify.

Family stories can still be meaningful without becoming definitive scientific explanations. As our discussion of reconstructive memory explores, siblings can also remember their shared past differently. Describing someone as “the quiet one” may capture a familiar pattern while overlooking context and development.

Individuality emerges through interconnected biological, psychological and environmental processes. Sharing a beginning does not require sharing a destination. Nor does becoming different prevent siblings from remaining close: people can recognise a common history while continuing to develop lives that are distinctly their own.

Written by Arkadiusz Dawidowicz · Developmental Psychology.

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