The study involved 10,845 males and females born during 3-9 March 1958 in England, Scotland, and Wales. The team investigated the combined effect of birth weight and socioeconomic environment on cognitive tests and educational achievements at 7, 11, 16, and 33 years.
All cognitive tests and educational achievements improved significantly with increasing birth weight. For example, the proportion of men with higher qualifications increased from 26% in the lowest (2500 g or less) birth weight group to 34% in the highest (more than 4000 g). For women, equivalent percentages were 17% and 28%. Standardised maths scores increased with increasing birth weight at all ages.
Social background had a strong effect on maths scores, with children from class I and II gaining higher scores than those from class IV and V. Looking jointly at the effects of birth weight and social class, participants of low birth weight from class I and II had higher average scores for maths than participants of normal birth weight from class IV and V. The association between maths score and social class seemed to strengthen with age, whilst the association with birth weight remained similar with age.
"Our results suggest a cumulative effect of prenatal (birth weight) and postnatal (social class) influences on cognitive development," say the authors. "Although the overall effect size of differences in cognitive scores associated with birth weight is small for individuals, the impact in populations may be important."
The greater explanatory value of social background suggests that gains in cognitive development may depend more on efforts to redress disadvantages in childhood social environment, they conclude.