Psychology › Personality & intelligence › Heritability: nature and nurture
Twin and adoption studies
Twins reared apart, 1979· Finding or experiment
Well replicatedStrong, repeated evidence.
Studies that compare identical and fraternal twins, or adopted children with both sets of parents, to separate genetic and environmental influences.
See it on the Psychology map →What it says
Identical twins share nearly all their DNA; fraternal twins share about half their varying genes, like ordinary siblings. If identical twins are more alike on a trait, genes likely contribute. In 1979 Thomas Bouchard began the Minnesota Study of Twins Reared Apart, starting with the 'Jim twins,' separated as infants and reunited at 39 with strikingly similar habits. Such studies show most traits are partly heritable, and that growing up in the same home does surprisingly little to make adult siblings' personalities alike.
Why it matters
They established that genes matter for nearly every trait and showed that shared family environment matters less for adult personality than most people expect.
The evidence
Large twin registries in Scandinavia, Australia and the US; a 2015 meta-analysis of 14.5 million twin pairs. The Cyril Burt scandal, in which a prominent researcher's twin data were apparently fabricated, prompted stricter scrutiny.
Open questions
Critics question the 'equal environments' assumption that identical twins are not treated more alike than fraternal twins, and DNA studies find lower heritability than twin studies.
Connections (2)
▸Twin and adoption studies estimate Heritability: nature and nurtureWell replicated
The connection
Twin studies turn comparisons into heritability estimates. A rough rule: double the gap between how alike identical twins are and how alike fraternal twins are. If identical twins correlate 0.8 on a trait and fraternal twins 0.5, heritability is about 0.6. Adoption studies offer a cross-check.
Who showed it and when
Galton suggested the twin method in 1875; large twin registries followed from the mid-20th century in Scandinavia, Australia and the US.
The evidence
A 2015 meta-analysis of 2,748 twin publications found average heritability near 49 percent; DNA-based methods confirm genes matter but give lower numbers.
Open questions
Whether identical twins' more similar treatment inflates estimates, and why DNA studies capture less than twin studies, the 'missing heritability' problem.
▸Twin and adoption studies show about 40% heritability in The Big Five personality traitsWell replicated
The connection
Twin and adoption studies find that about 40 percent of the variation in each Big Five trait is linked to genes. Surprisingly, shared home environment adds almost nothing; the rest comes from experiences unique to each person, plus measurement error.
Who showed it and when
Studies by Thomas Bouchard in Minnesota, John Loehlin in Texas and Kerry Jang in Canada, plus twin registries in Germany and Australia; a 2015 meta-analysis by Croatian researchers Vukasović and Bratko.
The evidence
The 2015 meta-analysis, pooling dozens of twin, family and adoption studies, put average heritability at about 40 percent.
Open questions
Why siblings raised together end up barely more alike in personality than if raised apart, and why DNA studies find only small genetic signals.