Cognitive Connie
Understanding the biological domain of personality
Behavioural genetics has generated some of the most robust and counterintuitive findings in all of psychology. Twin studies, adoption studies, and molecular genetic research converge on a consistent picture: personality is roughly 40–60% heritable; shared family environment — being raised by the same parents in the same home — accounts for surprisingly little variance in adult personality; and the most important environmental influences are those that differ between siblings raised together.
Key figures
Sandra Scarr & Kathleen McCartney
Scarr 1936–2014Proposed the three types of gene-environment correlation (passive, evocative, active) and argued that as children grow older and gain autonomy, active rGE (niche-picking) becomes increasingly important, explaining why shared environment effects diminish across development.
Thomas Bouchard
1937–presentLed the Minnesota Study of Twins Reared Apart (MISTRA), providing the most dramatic evidence for genetic contributions to personality by showing that MZ twins raised in different families are nearly as similar in personality as those raised together.
Robert Plomin
1948–presentBehavioural geneticist who has produced foundational work on heritability of personality and intelligence. Co-author of Behavioral Genetics (the field's standard textbook). Highlighted the importance of nonshared environment and the surprising weakness of shared environmental effects.
Avshalom Caspi & Terrie Moffitt
Caspi 1962–present; Moffitt 1955–presentPublished influential research on gene-environment interaction (Caspi et al., 2003) using the Dunedin longitudinal cohort, examining how genetic variants moderate the effects of early adversity on personality and psychopathology.
Key concepts
Heritability (h²)
A population-level statistic: the proportion of observed variance in a trait within a particular population that is attributable to genetic differences between individuals. Not a statement about any single individual. Can vary across populations and environments.
Twin study design
Compares monozygotic (MZ, identical) twins (who share ~100% of DNA) with dizygotic (DZ, fraternal) twins (who share ~50%). Greater MZ than DZ similarity in a trait implies genetic influence. The equal environments assumption — that MZ and DZ pairs are exposed to equally similar environments — is critical.
Adoption study design
Examines personality resemblance between adoptive parents and adopted children (which would reflect shared environment) and between adopted children and their biological parents (which would reflect genetic transmission). Consistently finds near-zero resemblance to adoptive parents but significant resemblance to biological parents.
Shared environment (c²)
Environmental factors that make siblings similar — same parents, home, neighbourhood, socioeconomic status. Despite its intuitive importance, shared environment accounts for very little variance in adult personality (typically <10%), one of the most surprising findings in behavioural genetics.
Nonshared environment (e²)
Environmental factors that differ between siblings raised in the same family — different peers, differential parental treatment, birth order effects, chance events. The largest environmental contributor to personality variance in adulthood. Includes measurement error.
Minnesota Study of Twins Reared Apart (MISTRA)
Bouchard et al. (1990): MZ twins raised in different families showed nearly as much personality similarity as MZ twins raised together (r ≈ .50 vs .52), demonstrating that shared family upbringing adds little to personality similarity and strongly supporting genetic transmission.
Gene-environment interaction (G×E)
The effect of a genetic variant on a trait depends on the environmental context. Example: Caspi et al. (2003) found that 5-HTTLPR (serotonin transporter) genotype moderated the effect of stressful life events on depression, illustrating that genes and environment can interact rather than simply adding together.
Gene-environment correlation (rGE)
The non-random association between genotype and environment. Three types: passive (parents provide both genes and environment), evocative (traits elicit reactions from others), active/niche-picking (people select environments congruent with their traits). Makes clean separation of genetic and environmental effects difficult.
Reaction range
Genes establish a range of possible phenotypic outcomes; the environment determines where within that range an individual ends up. A genotype does not fix a personality score but sets upper and lower limits modulated by environmental quality.
Epigenetics
Mechanisms (DNA methylation, histone modification) that regulate gene expression without changing DNA sequence, and can be influenced by experience. Of growing interest for understanding how early adversity may alter gene expression in ways that shape personality and vulnerability to psychopathology.
Test your knowledge
Frequently asked questions
How heritable is personality?+
Meta-analyses of twin studies (e.g., Vukasović & Bratko, 2015) find that Big Five personality traits are approximately 40–60% heritable. The remaining variance is primarily attributable to nonshared environment (unique experiences that differ between siblings). Shared family environment — being raised together — accounts for less than 10% of personality variance in adulthood.
Why does shared family environment have so little effect on adult personality?+
This is one of the most puzzling findings in behavioural genetics. Plomin and Daniels (1987) highlighted it in their paper "Why are children in the same family so different from one another?" Possible explanations include: sibling differentiation (deliberately carving out distinct niches within the family), different peer exposures outside the family, random developmental events, and gene-environment correlation (siblings with different genes elicit different environments even from the same parents).
Sources
Last reviewed July 2025- 1.
Eysenck H.J. (1967). The Biological Basis of Personality. Charles C. Thomas.
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Foundational neurobiological account of personality, linking extraversion and neuroticism to cortical arousal and the autonomic nervous system.
- 2.
Gray J.A. (1982). The Neuropsychology of Anxiety: An Enquiry into the Functions of the Septo-hippocampal System. Oxford University Press.
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Proposed the BIS/BAS model of personality, relating anxiety-related traits to limbic system function.