Relationship between family income and child asthma: an econometric perspective
Recent government reports in the United Kingdom (UK) and other countries have called for the implementation of policies and programmes to reduce socioeconomic inequalities in adverse child health outcomes. A number of observational studies conducted in North America have demonstrated a positive association between family income and child health outcomes, with some suggestion of a steepening of the income gradient during the later childhood years. Other health econometric studies in this area, most notably from the UK and other European countries, point to a much weaker income effect upon child health throughout the childhood years.
The conflicting evidence in this area motivates a further investigation into the extent to which the relationship between family income and child health might be spurious and driven by unobserved heterogeneity rather than representing a causal effect. Our study attempts to disentangle this nexus in the UK context using the instrumental variables approach, which will enable us to draw causal inferences by eliminating biases associated with unobservable factors.
Our focus was upon adverse childhood respiratory outcomes, principally preschool asthma, and was justified as follows. We examined data from the Millennium Cohort Study, a nationally representative birth cohort study collecting information on health, wealth, education, family and employment from parents of almost 19,000 children born in the UK in 2000-2001. Our analyses were undertaken using adverse respiratory health outcomes as dependent variables and family income as an independent variable. All models were adjusted for relevant controls, such as family history of asthma, smoking, breastfeeding and sociodemographic factors. Innovatively for the literature on respiratory diseases, we accounted for the endogeneity of the income variable by estimating probit models with instrumental variables. Appropriate instruments that were correlated with family income and simultaneously exogenous to adverse respiratory health outcomes were included in the models. A paper reporting the results of this study was published in Social Science and Medicine.