Introduction

When considering factors which act as major influences on fertility, religion and consanguinity may feature prominently, especially among the populations of less developed nations. Unfortunately, in many such studies it has been the practice either to assess the effects of religion and consanguinity separately, or to ignore consanguinity altogether. For the investigation of possible inter-relationships between the three parameters South India provides an excellent study centre. In the four southern states, Andhra Pradesh, Karnataka, Kerala and Tamil Nadu, with a combined population in the 1981 Census of India of 164.1 millions, consanguineous marriages are strongly favoured and three major religions, Hinduism, Islam and Christianity, are practised. Therefore detailed studies into the relative roles and effects of religion and consanguinity on fertility are possible at local, state and regional levels.

The study population

Data have been collected on fertility and a variety of fertility-associated parameters in the cities of Bangalore and Mysore, the present and former capital of the state of Karnataka, as part of a prospective neonatal screening programme for the detection of inherited amino acid disorders (Appaji Rao et al, 1988). Within three to five days of the birth of a child, details are obtained by interview from the mother on her age, the degree of genetic relatedness between husband and wife, their religion(s), and the number of liveborn and living children in the family. The consanguinity classes noted are: nonconsanguineous (with a coefficient of inbreeding in the offspring F=O), beyond second cousin (F < 0.0156), second cousin (F= 0.0156), first cousin (F= 0.0625) and uncle–niece (F=0.125).