Ideas about which factors have promoted gains in survivorship change over time. When the question first intrigued them, in the 1950s, scholars usually assumed that medical and public health improvements accounted for the mortality decline. During the 1960s, in the midst of impressive gains in material wealth in developed countries, many began to argue that economic growth had played a large, even a determinant, role. Each explanation, once subjected to the critical scrutiny that is typical of scholarly investigation, proved inadequate. It explained part but not all of the effect. So the search has continued.

In the late 1970s John Caldwell suggested a new idea. It was that the acquisition of literacy played a major role in controlling survival hazards for the children of parents who had become literate. The evidence for this explanation derived not from the developed countries, as earlier explanations had, but from developing countries. There, it could be shown, a parent's educational status revealed important things about the likelihood that the children of that family would survive. That effect usually held up even when quantitative and qualitative analyses also considered wealth and occupation, urban-rural residence, access to medical and public health services, and other factors believed to be important. Thus the education of young people to the point of literacy and perhaps to some distance beyond that, emerged as a means of enhancing survival even though the effects showed up more strongly in the next generation than in the current one.