Twins and IQ: Do Twins Have Lower IQs Than Singletons?
Twins IQ scores ran about 4 points below singletons in older studies, but recent cohorts in Denmark, the Netherlands and Norway show gaps of 0 to 2 points.
Dr. Russell T. WarneChief Scientist
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Twins have historically scored a little lower on IQ tests than singletons, by about 4 points on average in a meta-analysis covering more than 30,000 twins. In twins born in wealthy countries since the 1980s, that gap has largely closed, and identical twins turn out to be almost as alike in IQ as one person tested twice, typically differing by around 6 points.
Twins matter to intelligence research for a second reason: comparisons of identical and fraternal twins are the main way scientists estimate how much genes and environment contribute to IQ differences. This page first looks at the twin–singleton gap and why it has shrunk, then at how similar twins' scores really are, including twins raised in different homes.
Do twins have lower IQs than singletons?
The older evidence is consistent. In Birmingham, England, R. G. Record and colleagues compared verbal reasoning scores from the eleven-plus examination for children born in the 1950s. The 2,164 twins averaged 95.7 and 33 triplets averaged 91.6, against 100.1 for 48,913 single-born children. Ian Deary and colleagues found a similar deficit of about 5 IQ points among twins in two whole-population Scottish surveys of 11-year-olds, from 1932 and 1947. Father's occupation, overcrowding at home, childhood height and school attendance did not account for it.
Comparing twins with their own non-twin brothers and sisters removes family background as an explanation. In the Aberdeen Children of the 1950s cohort, Ronalds and colleagues found that twins scored 5.3 points lower than their singleton siblings at age 7 and 6.0 points lower at age 9.
Martin Voracek and Tanja Haubner pooled the published studies in a 2008 meta-analysis covering more than 30,000 twins and nearly 1.6 million singletons. Their best estimate of the gap was 4.2 IQ points, but it differed sharply by birth cohort: 5.1 points in older cohorts and 0.5 points in more recent ones.
Why the gap existed, and why it has shrunk
Two explanations have been tested. The first is prenatal: twins are usually born earlier and lighter than singletons. When Ronalds's team adjusted for birth weight and gestational age, the within-family gap fell to 2.6 points at age 7 and 4.1 points at age 9, with confidence intervals that included zero. Our article on IQ and premature birth explains why early delivery matters for later development.
The second is postnatal: two babies of the same age share parental time and attention. Record's team found an elegant test. Twins whose co-twin was stillborn or died within four weeks of birth, and so grew up as single children, averaged 98.8, only a little below the comparable singleton figure of 99.5. The authors concluded that the deficit came mainly from the twins' environment after birth.
Recent cohorts show much smaller gaps:
• Denmark: Kaare Christensen and colleagues compared all 3,411 twins born in 1986 to 1988 with a random sample of 7,796 singletons. Ninth-grade test scores were identical on average (8.02 for both), even though the twins had weighed 908 g less at birth.
• The Netherlands: Dinand Webbink and colleagues compared about 6,000 twins with about 188,000 singletons in primary school between 1994 and 2003. At age 6, twins scored 16% of a standard deviation lower in language and 17% lower in arithmetic, but those school-test gaps were no longer statistically significant at ages 10 and 12, and adult twins scored no differently from their own non-twin siblings.
• Norway: in 445,463 male conscripts born from 1967 to 1984, Willy Eriksen and colleagues found that singletons scored 0.11 standard deviations higher than twins, about 1.7 IQ points. The gap was the same within families, and adjusting for birth weight strongly reduced it.
Not everyone agrees the gap is gone. A 2025 preprint by Plessen, Tran and Voracek, which updates the earlier meta-analysis with 117 studies, estimates a gap of about 4 IQ points that is stable over time. It has not yet been peer reviewed, so the safest reading is that the twin deficit is small in recent cohorts from wealthy countries and that its size elsewhere is unsettled.
The adult evidence also matters for research. Danielle Posthuma and colleagues gave the Dutch WAIS-III to adult twins and their non-twin siblings and found no difference in cognitive performance, which supports applying twin-study results to everyone else.
How similar are identical twins' IQs?
Identical (monozygotic) twins share all their DNA, while fraternal (dizygotic) twins share about half of the genetic variants that differ between people, like ordinary siblings. Thomas Bouchard and Matt McGue reviewed 111 studies of family resemblance in 1981. As summarised by Claire Haworth and colleagues, the average IQ correlation was 0.86 for identical twins across 34 studies (4,672 pairs) and 0.60 for fraternal twins across 41 studies (5,546 pairs). For comparison, the same person retaking a test of general ability correlates with their own earlier score at about 0.90.
Twin similarity changes with age. In Haworth's study of 11,000 twin pairs from four countries, identical-twin correlations were 0.74 in childhood and 0.82 in young adulthood, while fraternal-twin correlations fell from 0.53 to 0.48. Heritability estimates rose from 41% at age 9 to 66% at age 17. Bouchard later called this rise the Wilson effect and estimated that the heritability of IQ levels off at about 0.80 by ages 18 to 20.
A correlation can be turned into a typical difference in IQ points. If scores are normally distributed with a standard deviation of 15, the average gap between two people whose scores correlate at r is about 0.8 × 15 × √(2 × (1 − r)). That gives:
• Identical twins (r = 0.86): about 6 IQ points apart on average.
• Fraternal twins (r = 0.60): about 11 points apart.
• Two unrelated people (r = 0): about 17 points apart.
• One person tested twice (r = 0.90): about 5 points apart.
So identical twins usually land close together, but a 10-point difference between them is not unusual, and some pairs differ by more.
What twins reared apart show
The most famous twin study looked at twins who grew up in different homes. Since 1979, the Minnesota Study of Twins Reared Apart put more than 100 sets of twins or triplets separated in infancy through a week of intensive psychological and physiological assessment. Bouchard and colleagues reported in Science in 1990 that about 70% of the variance in IQ was associated with genetic variation, close to the figure from smaller earlier studies. Identical twins reared apart were about as similar as those reared together on many measures of personality and interests.
The same authors stressed that a high heritability does not reduce the value of parenting or education, and suggested that genes may work largely by shaping the environments children experience. Heritability describes the sources of differences within a population and says nothing fixed about any one person. For the broader evidence on genes and intelligence, see our article on whether genes are important for determining intelligence.
Frequently asked questions
Do twins have lower IQs than singletons?
In older generations, yes, by about 4 to 5 points on average. Large studies of twins born in Denmark, the Netherlands and Norway since the late 1960s find gaps between 0 and about 2 points.
Do identical twins have the same IQ?
Not exactly. Their scores correlate at about 0.86, which works out to a typical difference of around 6 points, only slightly more than the roughly 5-point difference you would expect if one person took a test twice.
Do fraternal twins have similar IQs?
About as similar as ordinary brothers and sisters. Their scores correlate at about 0.60 on average, a typical difference of around 11 points.
Are twins reared apart still similar in IQ?
Yes. The Minnesota Study of Twins Reared Apart found that identical twins raised in different homes were strongly similar, with about 70% of IQ variance associated with genetic differences.
Should parents of twins worry about development?
Recent cohorts suggest that most twins catch up with singletons by the end of primary school. If a twin was born very early or very small, or parents have concerns, a paediatrician or health visitor can assess development.
The takeaway
The twin IQ deficit was real in children born in the mid-20th century, around 4 to 5 points, and seems to have come from earlier, lighter births and from sharing parental attention. In recent cohorts from wealthy countries the gap is small or absent by adolescence, although a new preprint argues otherwise. Identical twins are remarkably alike in IQ but not identical, typically about 6 points apart. If you want to see where your own score falls, you can take the RIOT IQ test.
References
1. Voracek, M., & Haubner, T. (2008). Twin-singleton differences in intelligence: A meta-analysis. Psychological Reports, 102(3), 951-962. doi.org
2. Record, R. G., McKeown, T., & Edwards, J. H. (1970). An investigation of the difference in measured intelligence between twins and single births. Annals of Human Genetics, 34(1), 11-20. doi.org
3. Deary, I. J., Pattie, A., Wilson, V., & Whalley, L. J. (2005). The cognitive cost of being a twin: Two whole-population surveys. Twin Research and Human Genetics, 8(4), 376-383. doi.org
4. Ronalds, G. A., De Stavola, B. L., & Leon, D. A. (2005). The cognitive cost of being a twin: Evidence from comparisons within families in the Aberdeen children of the 1950s cohort study. BMJ, 331(7528), 1306. doi.org
5. Christensen, K., Petersen, I., Skytthe, A., Herskind, A. M., McGue, M., & Bingley, P. (2006). Comparison of academic performance of twins and singletons in adolescence: Follow-up study. BMJ, 333(7578), 1095. doi.org
6. Webbink, D., Posthuma, D., Boomsma, D. I., de Geus, E. J. C., & Visscher, P. M. (2008). Do twins have lower cognitive ability than singletons? Intelligence, 36(6), 539-547. doi.org
7. Eriksen, W., Sundet, J. M., & Tambs, K. (2012). Twin-singleton differences in intelligence: A register-based birth cohort study of Norwegian males. Twin Research and Human Genetics, 15(5), 649-655. doi.org
8. Plessen, C. Y., Tran, U. S., & Voracek, M. (2025). One century of research on twin-singleton differences in intelligence: A preregistered meta-analysis and systematic review [Preprint]. PsyArXiv. doi.org
9. Posthuma, D., de Geus, E. J. C., Bleichrodt, N., & Boomsma, D. I. (2000). Twin-singleton differences in intelligence? Twin Research, 3(2), 83-87. doi.org
10. Bouchard, T. J., Jr., & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212(4498), 1055-1059. doi.org
11. Haworth, C. M. A., Wright, M. J., Luciano, M., Martin, N. G., de Geus, E. J. C., van Beijsterveldt, C. E. M., ... Plomin, R. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15(11), 1112-1120. doi.org
12. Bouchard, T. J., Jr. (2013). The Wilson effect: The increase in heritability of IQ with age. Twin Research and Human Genetics, 16(5), 923-930. doi.org
13. Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978), 223-228. doi.org
Hero image: Siamese twins cherries, by Stuhlfauth Thomas, CC BY 4.0. Via Wikimedia Commons.
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