It has been seen that fertility declines in the past have bee due to a multiplicity of inter-related factors.
It has, however, if been possible to identify precisely the contribution of each factor to the decline in fertility, or to understand the causal influences or to bind the factors in a systematic classificatory scheme.
The is no satisfactory explanation for the existence of differential fertility in developing countries due to factors not associated with economic development.
Demographers and social scientists are even today in search of a systematic theory, which would provide explanations for changes in fertility levels and class differentials in fertility, and which would also serve as a basis for predicting future fertility trends.
This gap in the knowledge of demographic phenomena continues, despite the efforts made by several social scientists to propound various theories of fertility.
In the following sections, the various theories of fertility are reviewed and classified into four categories: (1) Biological theories, (2) Social or cultural theories, (3) Economic theories, and (4) Socio-economic theories.
Of the four types of theories of fertility, the biological theories have been discussed in the chapter on Population Theories. As these biological theories of fertility are no longer accepted, they are of interest purely from the historical point of view.