Do Twins Have Lower IQ Than Singletons? What the Data Shows

Twins are sometimes said to score lower on IQ tests than children born singly, an idea that shows up often enough in casual writing about twins that it has the feel of settled fact. It has real research behind it, and the honest answer is more interesting than a flat yes or no: the gap was real in older studies, has shrunk sharply across generations, and the best-controlled recent evidence finds it has nearly disappeared — reversing for one measure entirely. None of this is the same question as whether IQ is heritable, which twin studies also answer, using a completely different comparison.

The question is more than academic curiosity. Twin births have become substantially more common in many countries over the past few decades, largely because of fertility treatment and a rising average age at childbirth, both of which raise the odds of a multiple pregnancy. A claim that applied to a small share of families a century ago now touches a meaningfully larger one, which is part of why it is worth checking against the actual data rather than repeating what sounds plausible.

What does the actual research find?

The most-cited UK study on this, published in the BMJ in 2005, compared 236 twins with 9,832 of their own singleton relatives across 8,160 families in the Aberdeen Children of the 1950s cohort. Comparing children to their own relatives, rather than to unrelated singletons, is a deliberate design choice: it holds much of the family's shared genetics, income and home environment constant, so whatever difference remains is less likely to be explained away by "twins happen to be born into different kinds of families." Before any adjustment, twins scored 5.3 points lower at age 7 (95% confidence interval 1.5 to 9.1) and 6.0 points lower at age 9 (95% CI 1.7 to 10.2). Adjusting for birth weight and gestational age — both typically lower in twin pregnancies — cut the age-7 gap to 2.6 points and the age-9 gap to 4.1 points, and further adjustment for sex, mother's age and number of older siblings made little additional difference.

That study tested children only at ages 7 and 9, so it does not by itself show a gap closing with age — and its own adjusted numbers did not shrink between those two ages. Claims that "the gap disappears by adulthood" have to come from comparing different studies of different generations, not from watching the same children grow up, which is a distinction worth being precise about.

Has the gap shrunk over time?

A 2008 meta-analysis pooling six countries and more than 30,000 twins against roughly 1.6 million singletons put the overall gap at 4.2 IQ points — describing it as less than one-third of a standard deviation — but split by era, studies of children born earlier in the 20th century showed a 5.1-point gap against just 0.5 points in more recently born cohorts. A separate UK study of 178,599 state-school pupils tested in 2004, at a mean age of 11, found the difference had become negligible: roughly 1% of a standard deviation on general cognitive ability and 3% on verbal reasoning, changes too small to matter in practice.

That pattern points to an era effect — better prenatal and obstetric care for twin pregnancies over the decades — rather than proof that any individual child's gap narrows as they get older. It is the same kind of generational shift our piece on the Flynn effect and expiring test norms covers for IQ scores broadly: population-level conditions change scores across birth cohorts in ways that have nothing to do with any single child aging.

The newest evidence: twins against their own siblings

The best-controlled study yet, published in Child Development in 2026, compared 1,702 twins with 851 of their own younger singleton siblings in the same households — 2,553 children in total from the UK's Twins Early Development Study — which rules out family-level differences like household income or parental education that complicate twin-versus-stranger comparisons. Measured at ages 2, 3, 4 and 7, twins scored lower than their singleton siblings on language, cognition and social-emotional measures at most ages, with small-to-moderate effect sizes. Language was the exception: by age 7, twins scored higher than their own singleton siblings.

That reversal is a reason not to compress this into a single "twins catch up" headline. In the most rigorously controlled data available, one domain did not just close the gap — it flipped, which is a more specific and more interesting finding than either "twins are behind" or "twins are fine" on its own.

What would explain a gap at all?

The clearest evidence points to prenatal factors rather than anything about how twins are raised. In the 2005 BMJ study, adjusting for birth weight and gestational age — both characteristically lower for twin pregnancies, which run shorter and split resources between two fetuses — accounted for roughly half of the raw score gap. That is a mechanism grounded in the pregnancy itself, not in parenting, and it is consistent with a gap that shrinks as obstetric care for twin pregnancies improves. It is a different story from other viral claims about what shapes a score: our piece on the breakfast question that supposedly reveals low IQ checks a claim with no real research behind it at all, unlike the twin-singleton gap, which does.

Twin studies and heritability are a different question

Twin studies are also used for something else entirely: estimating how heritable a trait is. That method compares identical twins' resemblance to EACH OTHER against fraternal twins' resemblance to each other — never twins as a group against singletons. A 2015 analysis pooling 2,748 publications and about 14.6 million twin pairs put the average heritability across human traits at 49%. Our own guide to twin studies and IQ heritability covers that comparison in full, and our blog's piece on whether intelligence is inherited from mother or father looks at a related but separate genetics question. "Twin studies show IQ is heritable" and "twins score lower than singletons" are answers to two unrelated questions, built from two different comparisons — confusing them is the actual myth this article is checking.

For context on the sibling-structure question more broadly, our guide to whether birth order affects IQ covers a related family-structure claim, and a 5-to-6-point difference on a mean-100, standard-deviation-15 scale is the difference between roughly the 50th and 63rd percentile — worth seeing on our percentile calculator before deciding how large it feels.

Should a parent of twins be concerned?

Not on this evidence. Even the largest raw gaps reported in the older literature, 5 to 6 points before adjustment, are well within the normal range of variation between any two children and shrink further once birth weight and gestational age are accounted for. The most rigorous modern comparison — twins against their own younger siblings — found effects concentrated in early childhood and, for language specifically, no disadvantage at all by age 7. A single score at any one age, for a twin or a singleton, is a snapshot with its own margin of error, not a verdict; our guide to turning an IQ into a standard unit shows how small a handful of points actually is once it is placed on the full scale.

The twin-singleton IQ gap was real in older data, has shrunk across generations of improved obstetric care, and in the most tightly controlled recent study had nearly closed by age 7 — reversing for language specifically. The IQ Metrics test reports a score on the same mean-100, SD-15 scale referenced throughout this piece.

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