David B. discusses The Unread Fisher, the last five chapters of R. A. Fisher’s Genetical Theory of Natural Selection, which are devoted to human evolution and especially to the evolution of fertility:
He argues that fertility, like any other trait, is subject to natural selection, and that the most important factor in determining optimal fertility is the amount of parental expenditure required: ‘In organisms in which that degree of parental expenditure, which yields the highest proportionate probability of survival, is large compared to the resources available, the optimal fertility will be relatively low’ (p.204).In ‘civilized’ man, the most important determinants of fertility are psychological. There are factors of temperament which determine the propensity to marry, whether marriage is early or late, and the degree of enthusiasm for children (p.210-13). But there are also social and institutional factors such as prohibitions on infanticide (p.218-21). These factors will themselves be affected by psychological influences which will vary over time, (p.219), since parents who are reluctant to commit infanticide will have more children surviving, and the children will tend to inherit their parents’ temperament.
Fisher argues that this is responsible for the changing historical views on infanticide, and minimises the role of religious doctrine, which itself (he argues) is responsive to the general mood of the population. In a splendidly Fisherian phrase he remarks: ‘It would, I believe, be a fundamental mistake to imagine that the moral attitude of any religious community is to any important extent deducible from the intellectual conceptions of their theology (however much preachers make it their business so to deduce it)‘ (p.222).