Birth Weight, Infant Mortality, and Causality

It is clear that birth weight (especially low birth weight) is associated with infant mortality and it is often assumed that birth weight lies on the causal pathway to infant mortality (the red path on the figure below). Alternatively, the Wilcox-Russell Hypothesis suggests that birth weight is not on the causal pathway to infant mortality and instead other factors influence both infant mortality and birth weight (the black paths on the figure below). A proximate determinants model (shown below) can help elucidate these relationships and help determine whether or not birth weight influences mortality or if it is only correlated with another exogenous variable that affects infant mortality directly.

The proximate determinants model for birth weigh and infant mortality.