Sunday, January 04, 2009

The Unread Fisher: Human Evolution (Part 2)   posted by DavidB @ 1/04/2009 03:56:00 AM

This note concludes my discussion of R. A. Fisher's neglected treatment of human evolution in The Genetical Theory of Natural Selection. In Part 1, here, I pointed out that Fisher believed that human evolution was continuing rapidly in the present day. He also argued that natural selection among humans now operated mainly through differences in fertility rather than mortality. It was therefore important for Fisher to show that such differences in fertility were heritable.....

The Inheritance of Human Fertility

Chapter 9 of GTNS deals with this subject. Fisher first provides data to show that the variance in human fertility is far higher than would be expected by chance, that is, if offspring were allocated to parents by a random process (p.207-9. As previously, I will give page references to the Dover edition). Some individuals have fewer, and some more, offspring than can be expected on a chance basis. He remarks that the non-random element in human reproductive variance appears to be much higher than in most other organisms (apart from the social insects, where some individuals specialise in reproduction). Among humans differences in fertility are also more important than differences in mortality (p.213). Fisher then discusses the factors leading to such large differences in fertility, and suggests that they are mainly differences of temperament rather than physiology (p.209-13). Fisher believes these differences largely innate (p.210-11), making it plausible that they are to some extent heritable. He then considers direct evidence of the inheritance of fertility, and concludes that over 40% of the observed variance is due to heritable causes (p.217). Unfortunately, his evidence for this rests entirely on studies of the British aristocracy. Subsequent studies of wider samples have usually produced lower estimates of heritability, sometimes effectively zero. However, a recent survey of the literature by M. Murphy suggests that the correlation between fertility of parents and offspring is not negligible, and may have been increasing over time. But little is known about the extent to which this correlation is due to genetic rather than cultural factors.

Social Differences in Fertility

Chapter 10 produces evidence from several countries that under modern conditions fertility is inversely related to social class: the higher the class, the lower the fertility. His evidence is reasonably strong, so I need not elaborate on it. He also makes a number of interesting comments:

a) he disputes the common belief that the class differential in fertility is due to the earlier adoption of birth control by the higher classes, and that the differential will disappear as the use of birth control spreads (p.239). Here Fisher has been proved partly right (in Britain, at least): the differential has not disappeared, but it has narrowed with the availability of modern contraceptives, and especially the Pill. Moreover, for much of the 20th century there was a 'U-shaped' fertility pattern, with the highest and lowest social classes more fertile than the lower-middles.

b) Fisher believes that the inverse correlation of class and fertility is comparatively recent in the modern West, but that it also existed in ancient Greece and Rome (p.241).

c) the inverse correlation is 'unnatural' in the sense that it has to overcome natural obstacles. Ordinarily, we would expect the wealthier classes to be more fertile, because they could delegate much of the burden of parental care (p.242).

d) the fertility differential means that a large amount of upward social movement is needed just to maintain the proportions of different occupational groups. Fisher has some interesting discussion of the effects of this on social attitudes and values (p.243-5). But he probably overestimates the importance of differential fertility as compared with economic and technological development. There has been a huge increase in the proportion of non-manual jobs in modern economies, which cannot be explained by differential fertility.

The Social Selection of Fertility

To explain the social class fertility differential Fisher appeals to what he calls the 'social selection' of fertility (p.250-6). (He gives credit for the basic theory to the little-known eugenist J. A. Cobb.) The key point of the theory is that in modern societies social class is influenced both by natural ability and by the resources provided by parents to their offspring, such as paid education, jobs in family businesses, capital gifts and loans, influential social contacts, etc. Since the amount of resources available per child is greater when there are fewer children in the family, there is a social advantage in relative infertility. Given equal natural ability, children from small families are more likely to rise in social status (or avoid a decline) than those from large ones. The higher social classes will therefore become on average less fertile. Since marriage occurs mainly within social classes, the qualities correlated within each class (such as high abilities and low fertility in the upper classes) will become statistically and genetically linked. Fisher then proposes this as the main factor behind the decline of ruling classes and of civilisations (p.256-61). He discusses but rejects alternative explanations, and in particular Gobineau's theory that decline is due to racial mixture. Fisher points out that racial mixture increases genetic variance and therefore increases the intensity of natural selection, but whether this helps or harms the quality of the population (in 'virtue and ability') will depend on the prevailing conditions of selection. If they are unfavourable (dysgenic) racial mixture will accelerate the decline, but if they are favourable its long-term effects will be beneficial (p.257) despite possible short-term drawbacks.

Stages of Human History

Fisher argues that a negative correlation of social class and fertility will occur whenever social conditions are similar to those in modern western society; broadly, whenever society is based on 'individuals co-operating for mutual advantage in a state of law and order' (p.261). But many societies are not of this kind. Fisher particularly discusses what he calls (in a non-pejorative sense) 'barbarian' societies, such as those of the Homeric poems, where there is little central government and law, and social life is based on kinship and the institution of the blood feud. In such societies fertility is a positive social advantage, and infertility a drawback (p.261-4). The qualities recognised by the society as valuable therefore become positively correlated with fertility (p.264). Fisher considers this form of 'social selection' far more powerful in promoting 'the higher human faculties', such as aesthetic appreciation, than either individual or group selection (p.264). He then has a fascinating section on 'Heroism and the higher human faculties', in which he give a major role to sexual selection. Unlike some more recent writers, such as Geoffrey Miller, Fisher recognises that marriage choices in such a society are made not by individuals but by families: 'The prestige of the contracting parties is all-important, and while this is partly personal, it is also largely tribal' (p.266). Sexual selection therefore reinforces the advantages of such socially valued attributes as heroism, even beyond the point at which they are directly beneficial (p.266); an example of Fisher's famous 'runaway' process. Fisher himself summarises his theory as follows:

To summarize the points of anthropological importance: (i) a barbarian people organized in kindred groups and recognizing the blood feud as the principle of social cohesion, can scarcely fail to experience a selection in favour of two qualities on which the success of the kindred group principally depends (a) the public spirited, patriotic, or heroic disposition (b) fertility. (ii) The stratification of society in these two qualities implies a selective advantage of the heroic temperament beyond the optimum advantage ascribable to prudent boldness, by reason of the social advantage of fame or heroic reputation. (iii) The power of recognizing the heroic qualities, and of conscious choice in intermarriage, introduces the dual effect of sexual selection in intensifying both the qualities selected and the communal recognition and appreciation of such qualities. (iv) This selection of the popular emotional response to the heroic qualities has the important effects of (a) stabilizing the foundations of the system by strengthening the existing basis of social cohesion, (b) intensifying the selective advantage ascribable to fame or prestige, (c) increasing the selective advantage of all qualities consciously envisaged in sexual selection, (d) exaggerating the realities of natural inequality by the development of an extreme aristocratic doctrine of hereditary nobility.(p.268)

Overall, Fisher's theory of human evolution is subtle and ingenious, but often speculative. The evidence for some of his key propositions, such as the high heritability of fertility, is painfully thin. Nevertheless, Fisher's ideas are always intriguing, and even his wildest speculations are well worth reading. Indeed, although much of this part of GTNS inevitably seems dated, in some respects it still compares favourably with more recent treatments of human evolution. Along with Darwin's Descent of Man, and Westermarck's Origin and Development of the Moral Ideas, I believe it should have a place among those few 'classics' that are still capable of stimulating modern research on the subject.