The Minnesota Study of Twins Reared Apart found that about 70% of the differences in IQ between people were linked to genetic differences.1PubMed. Sources of human psychological differences: the Minnesota Study of Twins Reared Apart Its second striking result was that identical twins raised in different homes ended up about as alike in personality, interests and attitudes as identical twins who grew up together.2Taylor & Francis. Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart Published in 1990, the study challenged the common belief that upbringing is the main force shaping who a person becomes. Its methods have also drawn sharp criticism, and it remains one of the most cited and most argued-over studies in psychology.3Mad In America. Bad-Science Warning: The “Minnesota Study of Twins Reared Apart” (MISTRA)
What the 70 Percent IQ Figure Really Means
The researchers estimated the heritability of IQ, meaning the share of differences in scores across people that lines up with genetic differences, at about 70%.4Free Associations: Psychoanalysis and Culture, Media, Groups, Politics. The 1990 Minnesota Study of Twins Reared-Apart IQ Study Ripe for Retraction Heritability is a measure about variation in a group. It says nothing about how much of one person’s IQ comes from genes. Someone who scored 110 on an IQ test can’t say that 70% of those points came from genes and the rest from experience.
The 70% estimate agreed with earlier, smaller studies of identical twins raised apart.5PubMed. Sources of human psychological differences: the Minnesota Study of Twins Reared Apart The authors concluded that IQ is strongly affected by genetic factors, and they described genes as strongly influencing both behavior and general intelligence.6Free Associations: Psychoanalysis and Culture, Media, Groups, Politics. The 1990 Minnesota Study of Twins Reared-Apart IQ Study Ripe for Retraction7The Embryo Project Encyclopedia. “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart” (1990), by Thomas J. Bouchard Jr, David T. Lykken, Matthew McGue, Nancy L. Segal and Auke Tellegen The remaining roughly 30% of differences was linked to environment, life experience and measurement error. Measurement error arises because any IQ test gives slightly different scores for the same person on different days. The rest of that 30% is environment the twins did not share, such as different teachers, illnesses, friends and chance events, which makes twins differ even with identical genes.
Twins raised apart are useful here because identical twins share essentially all their genes, so separating them leaves genes as the main thing they have in common. If upbringing mattered most, separated twins should differ much more than twins raised together. Similarity between separated identical twins therefore points toward genes, or toward things genes help shape.
A high figure also does not mean intelligence is fixed or can’t be improved by schooling or nutrition. Heritability depends on how varied conditions are in the group, so it can change if environments change. Height is highly heritable, yet average height rose over generations as diets improved. And the figure can’t explain average differences between countries or groups, only differences within the group that was studied.
Twins Raised Apart Were Nearly As Alike As Twins Raised Together
On many measures of personality and temperament, work and leisure interests, and social attitudes, separated identical twins were about as similar as identical twins raised together.8Taylor & Francis. Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart Growing up in the same household added surprisingly little extra similarity for these traits.9Embryo Project Encyclopedia. Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart Similar does not mean identical, though; the twins still differed in many ways.
Similarity was usually expressed as a correlation, a number showing how closely one twin’s score predicts the other’s, rather than a count of matching answers. Each twin filled out the same questionnaires separately, so their answers couldn’t influence each other. Personality questionnaires asked about things like sociability, stress reactions and rule-following, giving each twin a profile that could be compared.
About 50% of the differences in people’s interests were linked to genetic variation, roughly two thirds of the part of interests that stays stable over time.10Journal of Applied Psychology. Heritability of Interests: A Twin Study Identical twins showed a correlation of about 0.50 in job interests, while fraternal twins showed about 0.10, slightly less than a parent and child.11UPI. Twin study uncovers genetic role in job, religous interests If shared family life drove similar interests, fraternal twins raised in one home would resemble each other about as much as identical twins raised together, yet they showed far less similarity.
About 50% of the differences in religious interests, attitudes and values was linked to genes.12Psychological Science. Genetic and Environmental Influences on Religious Interests, Attitudes, and Values: A Study of Twins Reared Apart and Together That surprises people because religion is usually thought of as taught by family and community. The finding concerns how religious or traditional a person tends to be, not which faith they follow. Family still shapes which tradition a child is raised in.
How the Minnesota Twin Study Worked
The study included adult twins separated very early in life, raised apart through their formative years, and reunited in adulthood.13The Embryo Project Encyclopedia. Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart By 2000 the sample included 81 identical pairs and 56 fraternal pairs raised apart.14Mad In America. Bad-Science Warning: The “Minnesota Study of Twins Reared Apart” (MISTRA) Identical twins come from one fertilized egg and share nearly all their genes. Fraternal twins come from two eggs and share about half their genes, like ordinary siblings. Comparing how alike each type is shows how much genes add beyond a shared birth and age: if identical pairs are much more alike than fraternal pairs, genes are likely involved.
The study was led by psychologist Thomas Bouchard Jr. with David Lykken, Matthew McGue, Nancy Segal and Auke Tellegen, and the main results were published in the journal Science in 1990.15Free Associations: Psychoanalysis and Culture, Media, Groups, Politics. The 1990 Minnesota Study of Twins Reared-Apart IQ Study Ripe for Retraction
Twins traveled to Minnesota for about a week of testing. They took intelligence tests, personality questionnaires and interest inventories, and medical and life-history interviews recorded how and where each twin was raised.
Such pairs are rare. Separating twins at birth became uncommon as adoption practices changed, and finding pairs took years of searching through media and agencies. Many came to the researchers’ attention through news stories, word of mouth and referrals, and some twins contacted the team themselves after learning of the project. Because participants largely volunteered, the sample may not represent all separated twins. Small numbers also limit how precise any estimate can be.
“Raised apart” meant separated early and growing up in different households, but the time before reunion and the amount of contact varied from pair to pair. Researchers recorded when twins were separated and when they met, so those details could be examined as possible influences on similarity.
Why Genes Might Make Separated Twins Alike
The authors argued that the different home environments did not have a significant effect on how separated identical twins developed, including their social attitudes.16The Embryo Project Encyclopedia. “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart” (1990), by Thomas J. Bouchard Jr, David T. Lykken, Matthew McGue, Nancy L. Segal and Auke Tellegen They proposed that genes influence how people respond to their surroundings, so twins with matching genes may focus on and hold on to similar experiences, which then makes them more alike.17The Embryo Project Encyclopedia. “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart” (1990), by Thomas J. Bouchard Jr, David T. Lykken, Matthew McGue, Nancy L. Segal and Auke Tellegen Environment still matters, but people partly choose and shape their environment based on inherited tendencies.
Genes can work through environment in three ways. In the passive route, parents give children both genes and a home that fits those genes, such as bookish parents with a bookish child, a link separated twins would not share. In the evocative route, a child’s inherited temperament draws particular reactions from other people, such as cheerful children getting friendlier responses. In the active route, children pick friends, activities and hobbies that suit their inclinations as they grow, and this is the route most relevant to twins raised in different homes.
None of this means parenting is irrelevant to health, safety or opportunity. Homes in the study were mostly within a normal range, not severe deprivation, and extreme neglect or poverty can still harm development.
Criticisms of the Minnesota Twin Study
A critic argues that IQ similarity among identical and fraternal twins raised apart did not differ by enough to be statistically reliable.18Mad In America. Bad-Science Warning: The “Minnesota Study of Twins Reared Apart” (MISTRA) That matters because a clear gap between the two groups is a key step in showing genes influence IQ.19Mad In America. Bad-Science Warning: The “Minnesota Study of Twins Reared Apart” (MISTRA) Some critics have called for the 1990 IQ paper to be retracted.20Free Associations: Psychoanalysis and Culture, Media, Groups, Politics. The 1990 Minnesota Study of Twins Reared-Apart IQ Study Ripe for Retraction These criticisms come from commentary pieces, not new studies, and the paper has not been retracted. The complaint concerns one comparison in the IQ analysis, so it does not by itself show that other findings, such as personality similarity, are wrong.
Other objections are common in studies of twins raised apart. Some pairs met or lived near each other before testing, which could add similarity. Adoption agencies often placed children in similar kinds of homes. Twins shared the same womb, an environment genes alone don’t explain. Volunteers who answered ads may differ from twins who never came forward. Critics say many twins had contact or similar placements, so they were not truly independent environments, while defenders say the typical differences between homes were still substantial.
How the Genetic Link to IQ Changes With Age
The share of IQ differences linked to genes increases with age.21Twin Research and Human Genetics. The Wilson Effect: The Increase in Heritability of IQ With Age It levels off at about 80% around ages 18 to 20 and stays there well into adulthood.22Twin Research and Human Genetics. The Wilson Effect: The Increase in Heritability of IQ With Age Over this period, identical twins grow more alike in IQ while fraternal twins grow less alike.
One reason may be that young children live in environments chosen by parents, while adults pick their own jobs, friends and hobbies. Those choices tend to follow inherited tendencies, so genetic influence shows more strongly.
The Minnesota study’s adult twins were tested at an age when genetic influence is near its peak. A figure like 70% therefore reflects a stage of life when the genetic link is relatively high, and a study of young children would likely find a lower number. That is one reason heritability estimates differ among studies, since each samples people at different ages. One analysis also suggests genetic influence on general thinking ability may decline again in advanced old age.23Personality and Individual Differences. The anti-Wilson effect: The decrease in heritability of general cognitive ability, as proxied by polygenic expressivity, with advanced age
Identical twins also become less identical over time. When young, they look almost the same in the chemical tags that switch genes up or down.24PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins Older identical twins show striking differences in these tags, changing which genes are active.25PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins The differences keep building from early adulthood to old age.26PubMed Central. Epigenetic variation during the adult lifespan: cross-sectional and longitudinal data on monozygotic twin pairs
Does the Finding Still Hold Up Today
A study drawing on 11,000 twin pairs from six studies in four countries found genetic variation contributes substantially to high intelligence.27PubMed Central. A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries The results were similar across different tests and samples, which strengthens the conclusion.28PubMed Central. A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries Minnesota twin data are also combined and compared with later Minnesota twin and adoption studies.29Cambridge University Press (Twin Research and Human Genetics). Minnesota Center for Twin and Family Research
“Holding up” has limits. Later work supports a large genetic role in intelligence overall, but the exact 70% figure varies between studies and age groups. DNA studies so far explain less than twin studies estimate, a gap researchers are still working on. No single gene for intelligence exists; many genes each add a tiny effect. Twin studies estimate the total genetic influence without needing to know which genes are involved, while genome scans look for which small DNA variations go with higher or lower test scores.30PubMed Central. Twin studies to GWAS: There and back again31PubMed Central. Twin studies to GWAS: There and back again32PubMed Central. Twin studies to GWAS: There and back again