In this week's BMJ, Christensen and colleagues1 investigate two questions that are of popular and medical-scientific interest. Firstly, do twins have lower intellectual skills and educational achievements than singletons and, secondly, is birth weight associated with intellectual and educational performance? The authors look at the second question in both twins and singletons. They also open up the question of the link between intelligence and education, because the main comparator studies used IQ-type outcomes rather than educational performance.2 3

The study uses the Danish registration system linked with the Danish demographic database, the national hospital discharge register, the register of compulsory school completion assessments and test scores, and the Danish twin registry. They therefore had data on standard national educational outcomes, birth weights, and other demographic and parental variables for the entire relevant population of twins, and for a large representative sample of the comparable population of singletons.

Firstly, do twins have lower intellectual skills and educational achievements? Three recent studies of large samples of Scottish children born between 1921 and the 1950s strongly suggest they do. …