The effect of breastfeeding on health and developmental outcomes: an instrumental variable approach

by

Emla Fitzsimons, Senior Research Economist, Institute for Fiscal Studies

Marcos Vera-Hernandez, Economics Lecturer, University College London

The causal effects of breastfeeding on children's outcomes such as health and cognitive development are little understood. The problem is that breastfeeding is a choice made by parents, and so any associations between breastfeeding and outcomes may be confounded by broader socio-economic factors. Even if we control for these as best we can, there is still the concern that there are unobserved factors that simultaneously affect the decision to breastfeed and children's outcomes.

If well conducted, randomized trials are known to provide the most robust evidence on causal effects. However, a breastfeeding randomized trial would probably be very costly and unethical. In the absence of a randomised controlled trial, economists have long turned to the method of instrumental variables in order to identify causal effects. It is a technique that recognises that breastfed and non-breastfed children would have different average outcomes even regardless of whether or not they were breastfed, most likely because they differ in background characteristics. This technique works by assumming that there is one variable, called an instrumental variable, that (1) predicts breastfeeding participation, (2) only affects outcomes through affecting breastfeeding, and (3) is not correlated with the unobserved background characteristics that affect outcomes. The credibility of the resulting estimate thus hinges on the instrument being suitable. We believe that day of birth satisfies all of these requirements. Our hunch is that it satisfies (1) because day of birth impacts on the amount of breastfeeding support/advice received whilst in hospital. For instance, the intensity of support available at weekends is generally less than that available during the week, which may subsequently affect breastfeeding rates. We have no reason to believe it has a direct effect on outcomes, thus satisfying (2) (a possible concern is with planned Caesarean sections, which we would exclude from the analysis). And because time of delivery is a fairly random event, we believe it satisfies requirement (3).

It is for this reason that we are seeking access to the data already collected by the ALSPAC study. It is an ideal data set to use to address our research questions: on the one hand it has extremely detailed information on pregnancy, labour, delivery and infant-feeding practices; and on the other, very detailed information on child measurement and assessment.

Apart from the information contained in the questionnaires and the child measurements, the methodology also requires access to the variable day of birth.

We anticipate that we could use many of the variables collected under the ALSPAC study. The information collected before birth will be used to check the suitability of the instrumental variable (day of the week should be uncorrelated with background variables). The longer the list of variables that we will present, the more credible our instrumental variable will be. We will also use information on child questionnaires and measurements to build our outcome variables (child's health and development). We will also use parents' information collected after birth to consider the pathways through which breastfeeding could affect the outcomes (for instance, whether parental investments in the child follow compensating or reinforcement behaviours)

At this preliminary stage of the research it is difficult to pin down exactly which variables we would use. Moreover, the results of the estimations will inform us on what other variables to use. For these reasons, ideally we would like to have access to :(1) day of birth, (2) all variables in the child and parents questionnaires collected at any moment in time, (3) assessments (4) linkage with school data, (5) variables that have been derived from biological samples at any moment in time