Nathan Keyfitz was one of a very small group – Raymond Pearl, Alfred Lotka, Ansley Coale, Mindel Sheps, Jean Bourgeois Pichat, William Brass - renowned for their applications of mathematics to understanding population issues. Keyfitz and Sheps, both Canadians, were born in Montreal and Winnipeg respectively.

Sheps and I, following in their footsteps, developed mathematical models that illuminate the determinants of fertility and family building in both developing and developed countries. One of our first questions was why, comparing French Canadians to Indian and Bangladeshi women, none of whom used contraception, the French Canadians could average over a dozen children while South Asian women, in the same reproductive period, averaged under seven. We were able to consider the relative roles of both biological and social practices in controlling fertility. I will discuss these contributions of mathematics - some of them quite surprising - to understanding human fertility, how they provided direction for essential data collection, and some of my own later studies – in places as diverse as Bangladesh and Colorado – that they inspired.