Modelling diffusion effects in fertility transition

Abstract

Mathematical models bind theory to precise formulations, and by doing so accentuate logical inconsistences in theories and facilitate their testing. In this article we explore the implications of simple mathematical models of fertility transition that contain social interaction diffusion effects. We put forward the hypothesis that diffusion effects act as a third type of causal agent of fertility transition, above and beyond the traditionally studied demand and supply factors. Substantial historical and contemporary evidence points to the existence of diffusion effects on the timing and pace of fertility transition.1 Among the pieces of evidence most commonly cited are the fact that fertility declines have occurred under a wide variety of social and economic circumstances, with the pace of decline frequently outpacing the rate of socio-economic change; the irreversibility and rapid pace of decline; the remarkable spatial and temporal patterning of fertility decline in terms of linguistic, ethnic, and cultural boundaries; and, in contemporary developing societies, the relatively small cross-societal and temporal variation in stated reproductive preferences, as compared to the variation in levels of fertility and contraceptive behaviour in the same populations. While diffusionist explanations are now common in the literature, a lack of conceptual rigour blurs the debate on the role of diffusion dynamics in fertility transition. The possibility that information flows and the diffusion of ideas play a significant role in fertility transition is already mentioned in the earliest fertility studies. Caldwell2 complained that ‘in the late nineteenth and early twentieth centuries, governments and other institutions almost invariably explained fertility control innovationally, as the spread of pernicious ideas’. Among the earliest fertility transition analysts, Thompson3 explained the relatively low fertility of clerical workers in terms of the ‘more or less close proximity to their bosses ’. An emphasis on the role of the diffusion of ideas characterizes more recent work as well: Cleland and Wilson4 posited ‘ideational change’ as an alternative to explanations centred on material change, and Freedman5 described ideas and mental frameworks as quasi-independent forces that spread through national and world networks of communication and transportation.