The Biological Domain of Personality

All 10 Terms & Definitions

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.