Around the middle of the 20^th century, most western countries experienced a surge in birth rates, the “Baby Boom”. This was unexpected at the time, and the causes and underlying social mechanisms remain unclear. In this paper we suggest that a normative shift, propelled by the power of social interaction, might have been one of the main drivers of the Baby Boom, namely the emergence and diffusion of the two-child norm. If new types of childbearing behaviour emerge, diffusion via social networks can translate these changes on the micro level into large scale changes on the macro level. At the end of the 20th century, the two-child family clearly was a well-established norm. The roots of this trend go back to the 19th century fertility transition but it was during the Baby Boom era that the two-child family won a lot of ground. This normative shift of family size lead to higher birth rates as childlessness became rare. However, among regions where larger family sizes were still common, it contributed to a further fertility decline. The resulting homogenization could therefore explain similarities and dissimilarities across regions and social groups we observe in the country of our focus: Belgium.

Studying dynamic social influence processes in social network structures is difficult with standard demographic tools. Thus, to theoretically explore the mechanism that we propose, we make use of agent-based computational modelling. This will enable us to develop and explicate theories about social behaviour and assess the implications of their assumptions by means of computational simulation and experimentation. Using input data from Belgian censuses, we simulate the diffusion of the two-child norm among social and spatial dimensions and the resulting fertility outcomes. Preliminary results show that this diffusion process can indeed account for the variety of cohort fertility trends we observe.