The IFs demographic model captures the mechanical or accounting effects of mortality on population (see the solid paths in the figure below).  A key pathway passes from mortality through adult age population to labor supply (including aging-related lags). ^[1]   Similarly, IFs captures the mechanical effect of mortality on fertility through the death of women of childbearing age.

The most important non-mechanical linkage is almost certainly the relationship between child mortality and fertility. IFs forecasts fertility as a relationship with infant mortality, the log of educational level of those aged 15 and older (neither the education of women alone nor the education of those 15-24 work as well), and the percentage use of modern contraception.