Still unequal at birth: Birth weight, socioeconomic status and outcomes at Age 9

Birth weight is an important aspect of public health which has been linked to increased risk of infant death, increased cost of care, and a range of later life outcomes. Using data from a new Irish cohort study, I document the relationship between birth weight and socioeconomic status. A strong association with maternal education does not appear to be due to the timing of birth or complications during pregnancy, even controlling for a wide range of background characteristics. However, results do suggest intergenerational persistence in the transmission of poor early life conditions. A comparison with the UK Millennium Cohort Study reveals similar social gradients in both countries. Birth weight predicts a number of outcomes at age 9, including test scores, hospital stays and health. An advantage of the data is that I am able to control for a number of typically unmeasured variables. I determine whether parental investments as measured by the quality of interaction with the child, parenting style, or school quality mediate the association between birth weight and later indicators. For test scores, there is evidence of non-linearity. Boys are more adversely affected than girls, and I find that the effects of low birth weight (<2,500g) are particularly strong. I also consider whether there are heterogeneous effects by ability using quantile regression. These results are consistent with a literature which finds that there is a causal relationship between early life conditions and later outcomes.