When smaller families look contagious: a spatial look at the French fertility decline using an agent-based simulation model

Despite some disagreements about specific timing, it is now widely accepted that France was the first European country to experience a systematic decline in fertility, a decline that took place in a very distinctive geographical pattern. Whereas two areas of low birth rates (the Seine valley and the Aquitaine region) kept spreading, two 'islands' of high fertility (Bretagne and the Massif Central) shrank until they more or less disappeared in the early 1900s. In an attempt to provide a sensible explanation of this pattern, we build an agent-based simulation model which incorporates both historical data on population characteristics and spatial informaton on the geography of France, and allows us to study the role of social influence in fertility decisions. We assess how different behavioural assumptions and network topologies cause variations in diffusion patterns, using quantitative data on the Ecclesiastical Oath of 1791 to proxy for the impact of the Revolution. Analysis of several simulations shows that a combination of both endogenous and exogenous factors help to explain the way in which the diffusion took place and suggests some of the mechanisms through which this was materialised