Twins are not rare—in many countries about 4% of newborns are twins,1 and the low birth weight and increased rate of prematurity in twins is a cause for concern for both parents and healthcare providers. Research shows that European twins born more than 50 years ago had lower IQ and academic performance than singletons. In more recent cohorts, however, the cognitive disadvantage in twins compared with singletons seems to have vanished. The linked Taiwanese study by Tsou and colleagues (doi: 10.1136/bmj.a438) adds to this literature by suggesting that the cognitive cost of being a twin is also small in the more developed countries of Asia.2

Twin studies are commonly used in medical and public health research to disentangle the contributions of genetic and environmental factors to health. Comparison of the health trajectories of twins and singletons has also provided a critical test of the fetal origins hypothesis, whereby growth restriction in the third trimester (experienced to a strong degree by twins) is hypothesised to “programme” increased risk …