Fertility decline is commonly explained as a consequence of changing preferences, economic constraints, or cultural transformation. This paper argues that such explanations fail because they operate at an inappropriate analytical scale. Reproduction is not primarily the result of explicit individual decisions but an emergent outcome of structural constraints governing pairing, selection, and coordination. Building on a framework of scale-dependent causality, the paper shows that fertility stability depends on threshold conditions that limit the effects of selection in asymmetric systems. Historical societies maintained fertility not through strong preferences for children, but through structural mechanisms that constrained pairing and linked sexuality to reproduction. Modern societies have removed many of these constraints while preserving underlying asymmetries, resulting in a regime where selection is no longer balanced. The observed decline in fertility can thus be interpreted as a structural consequence of scale mismatch between individual decision models and system-level reproductive dynamics. This paper further explains why financial incentives tend to have limited effects on fertility: they operate at the level of individual decisions but do not alter the structural conditions determining pairing density.
Miloslav Grundmann (Thu,) studied this question.