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Introduction
For many years, researchers have been asking whether sharing life with a co-twin in the womb or after birth affects cognitive ability. 1 The question is not only of importance to researchers who are interested in twins but may help to explain the determinants of cognitive ability more generally. Childhood cognition is predictive of educational attainment, socioeconomic position, and health in adulthood and therefore has important social and public health implications. 2 â[euro]" 4
Most previous studies reported that twins have lower cognitive ability than singletons. In a very large study of children born in Birmingham, United Kingdom, between 1950 and 1954, twins had a deficit in verbal reasoning scores at age 11 of 4.4 points on average. 5 In the US collaborative perinatal project of hospital births delivered in 1959-65, twins scored lower in cognitive tests at 8 months, 4 years, and 7 years, although substantial loss to follow up had occurred by 7 years. 6 In a national sample of Australian schoolchildren born in the 1960s, singletons performed better than twins in tests of word knowledge, reading, and numeracy at ages 10 and 14. 7 Similarly, among 10 year olds in Stockholm born in 1953, singletons tended to have higher verbal ability and numerical test scores than twins. 8 Most recently, a study was reported that used the Netherlands twin registry to look at differences within families in cognition between 260 adult twins and 98 of their singleton siblings. This found no evidence for a difference in cognitive ability between singletons and twins in the same family. 9
Despite these various studies it is still unclear whether something intrinsic to the experience in the womb of being a twin is associated with a cognitive deficit. Maternal characteristics and other aspects of the postnatal family and socioeconomic environment are clearly different between twins and singletons 10 ; many of these aspects are known to be related to cognitive ability. 11 â[euro]" 15 Much of this potential confounding by familial factors can be dealt with by studying whether twins have a cognitive deficit compared with their singleton brothers or sisters in the same family. However, the only study to date to take this approach did not adjust for factors that vary between siblings...