Commentary: Birthweight and childhood cognition: the use of twin studies.

It has long been recognized by obstetricians and paediatricians that babies of lower birthweight experience an increased risk of some physical and mental morbidities soon after birth and in childhood. Additionally, over the past two decades, the formulation of the ‘Developmental Origins of Health and Disease’ (Barker) hypothesis has resulted in acceptance that pre-natal influences (usually captured by birthweight) have a much longer reach than previously anticipated. Much of this research has concentrated on somatic illness—cardiovascular disease, cancer, their risk factors—but it has more recently been recognized that cognitive ability (itself a risk factor for these outcomes) in childhood and beyond may also be influenced by early life characteristics. Indeed, Barker has suggested that the understanding of the causes and consequences of cognitive function will be one of the three big contributions from fetal origins research, the others being obesity and longevity. Previous studies have shown a positive association between birthweight and intelligence, in both singletons and twins. In singletons, there is a small but statistically significant association that appears not to be fully explained by potential confounding factors such as socio-economic position. Twin studies provide a particularly powerful study design in this regard because, in principle, they implicitly control for the preand post-natal environment. A comparison of dizygotic with monozygotic twins allows the genetic contribution to a given characteristic to be quantified. Previous twin studies have shown conflicting results, with some suggesting that there is a direct relationship between birthweight and IQ, whereas others indicate that this is not the case. Therefore, there is a need for further well-powered twin studies—such as that by Torche and Echevarria in the present issue—to further investigate the complex links among genetic factors, the preand post-natal environment and intelligence, in childhood and beyond. Torche and Echevarria describe a twin study using a large sample of individuals (n1⁄4 220 062 with 2474 twins) born between 1998 and 1999, and tested in fourth grade (around the age of 9 years). That the study was based in Chile (a middle-income country with wide social inequality) is particularly relevant in a literature dominated by cohorts drawn from affluent nations. The use of routinely collected administrative data—unusual in countries with a developing infrastructure—prevents recall bias in birthweight. This study is also to be commended in using different statistical methodologies that attempt to establish the impact of weight differences both in twin births (where a common genetic and extra-uterine pre-natal environment is assumed) and singletons. The authors test for an interaction between the pre-natal environment and post-natal opportunity (estimated by maternal education), and find that lower birthweight is more strongly associated with cognitive ability at the age of 9 years in children whose mothers have less education relative to those who were well educated. This adds to the debate about early childhood intervention to improve social and cognitive outcomes, suggesting that such initiatives may have the most impact on public health if they target children of mothers who have been relatively disadvantaged. Torche and Echevarria’s findings add to the increasing evidence that the ‘cognitive cost’ of being a twin seems to have disappeared in cohorts born more recently. Thus, in studies whose members were born between 1921 and the 1950s, twins scored about five IQ points (about one-third of a standard deviation) lower than singletons. However, more recent studies have shown this difference to disappear in modern European cohorts whose members have experienced improved social circumstances Published by Oxford University Press on behalf of the International Epidemiological Association