In order to improve our understanding of human fertility, its variations among different groups and causes of the variations, it is necessary to analyse the mechanisms through which socioeconomic variables influence fertility.
It is clear from the foregoing discussion that Kingsley Davis and Judith Blake tried to describe the factors which influence fertility directly with the help of “intermediate fertility variables.”
The intermediate fertility variables no doubt helped demographers in improving their knowledge about human fertility and fertility differentials. However, in quantitative fertility studies, the intermediate fertility variables could not be used.
John Bongaarts has also identified direct determinants of fertility which he called “proximate determinants of fertility.” According to John Bongaarts and Robert Potter, “the proximate determinants of fertility are the biological and behavioural factors through which social, economic and environmental variables affect fertility.”
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They are called proximate as they are nearest to the event of fertility. It is possible to study fertility differentials among various populations or trends in fertility levels of any country over a period of time by studying the variations in one or more of the proximate variables.
The proximate determinants of fertility can be classified in two groups, viz., (1) those influencing the length of the reproductive span and (2) those influencing the rate of child-bearing within the reproductive.
1. Proximate Determinants Influencing Length of Reproductive Span:
It is obvious that a girl becomes capable of bearing children only after menarche the first menstruation. Thus the menarche marks the beginning of the reproductive span or period and the menopause marks the end of the reproductive period.
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Since in Indian and most other societies, socially sanctioned child-bearing is limited only to married women, the marriage of the girl is the starting point of her reproductive period and the disruption of marriage (either by death of the husband Or divorce or separation) or menopause (onset of permanent sterility), whichever is earlier, is the end point of her reproductive span.
Again in India, a small percentage of girls is married before the onset of menarche. In such a situation, the onset of menarche is the beginning of the reproductive period.
2. Proximate Determinants Influencing the Rate of Child-Bearing within the Reproductive Span:
Within the reproductive span, the married and fecund woman reproduces at a rate inversely related to the average duration of the birth interval. High fertility is associated with short birth intervals and low fertility is associated with long birth intervals.
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The length or duration of her birth interval (interval from one birth to the next birth or from the last birth to the onset of permanent sterility or marital disruption) is determined by three important components, viz., (a) Post-partum infecundable period, (b) Conception wait and (c) Gestation period. Figure 8.3 illustrates the components of the birth interval.