Like most Western countries, 19th-century Belgium was marked by a series of developments and changes, including a decline in fertility rates. Against this backdrop, little is known about the socio-demographic history of the rural world. The rural areas of southern Wallonia, while similar from an economic, social, and cultural point of view, were nevertheless characterized by highly diverse trajectories of fertility decline. Despite a large amount of literature on the subject, the diversity of the processes still needs to be explored. Why does the decline in fertility occur according to such a different timing in similar areas? How can we explain the pioneering nature of certain isolated rural areas in terms of fertility decline, when the rural world is often presented as more conservative and reluctant to change? As part of an approach to adapting demographic behaviors to socio-economic constraints, this presentation will show that the fertility decline should be interpreted as a response by populations to structural and cyclical changes. The use of this response (and therefore the diversity of processes) would depend on the inherent characteristics of local populations and systems. We take advantage of the long tradition of statistics available in Belgium to analyse the fertility decline using two approaches. The first approach, conducted at a macro level, involves a detailed analysis of the socio-demographic systems of three rural regions in southern Belgium in the 19th century. We provide evidence of this great diversity within the rural world, which we then attempt to explain. The second approach, conducted at the individual level, identifies the individual strategies used to maintain living standards and the individual determinants of fertility in the 19th century. The latter is based on an unpublished corpus of individual data compiled from the administrative registers of two rural communities in southern Belgium.