With the race to decode the human genome, the prestige of research in biological genetics is higher than ever. Its incursion into demography has long been viewed with enthusiasm but also with reservations. Atam VETTA and Daniel COURGEAU here set out the problems associated with the heritability analysis promoted by behaviour genetics. Going back to the work of Fisher (1918), the authors examine the principles of this analysis and criticize the mathematical formulas it uses, which are assimilated by demographers despite being marred by algebraic error. Their argument also rests on the belief that individual behaviour is explained largely by the social, political and economic conditions in which individuals live. In conclusion they argue that this current of genetics, which emerged in the early twentieth century, is outdated in the age of the genome and thus cannot provide a legitimate model for the study of human behaviour.
Recently, a number of researchers have published articles in major demographic journals (Kohler et al.,. 1999; Foster. 2000; Morgan and King, 2001; Rodgers et al., 2001), arguing that the methods of quantitative genetics based on Fisher (1918), and the model fitting approach used by behaviour geneticists in particular, should be used to study demographic behaviour. Other demographers support this view, because they believe that it is necessary to consider the impact of behavioural genetics on demographic behaviour (Coleman, 2002; Hobcraft, 2002). The links between genes and human reproduction (fertility and other fitness traits) are also the subject of interdisciplinary studies. New books (Rodgers et al., 2000: Rodgers and Kohler, 2003) consider various questions in this field. Previously, the behaviour genetics approach has been used in a number of social science areas. It has been used for over 30 years in psychology (Herrnstein and Murray, 1994; Dunne et al., 1997; Segal and McDonald, 1998; for more references, see Capron et al.. 1999), gerontology (McGue et al., 1993), sociology (Lichtenstein et al., 1992), psychiatry (Kender et al., 2000), and so on. The Behavior Genetics Society and its journal Behavior Genetics are dedicated to research using this methodology. The central point of these studies is the claim that there is a genetic component in behavioural traits, and that the contribution of this component to the variance of the traits in the population can be measured. Demographic traits for which this claim is now being advanced include fertility, mating success, longevity, juvenile survival, divorce, etc. The psychological and medical traits include "intelligence" as measured by IQ scores (Pedersen et al., 1992), personality (DiLalla et al., 1996), alcoholism (Blum et al., 1990), smoking (Kender et al., 2000), homosexuality (Eckert et al., 1986), femininity (Bouchard and McGue 1990), morningness-eveningness (Hurr et al., 1998), aggression, hostility and anger (Gustavson et al., 1996), obesity (Brookman and Bevoral, 2002), soda or fruit juice intake (de Castro, 1993), etc.
Galton's (1869) nature-nurture division is illusory. The two effects cannot be separated for any human trait. We explain the genetic concepts used in behaviour genetics model fitting and the concept of heritability, as well as their deficiencies. We suggest another concept to study inheritance of a trait. Most human traits involve assortative mating, but behaviour geneticists use incorrect formulae when they fit models involving assortative mating (Capron et al., 1999). We explain why Fisher's (1918) formulae are wrong and discuss the algebraic error in Jinks and Fulker (1970). We enumerate some factors that affect fertility and give examples of how molecular biology and genomic research, and specifically the unravelling of the species' genome, are increasing our knowledge of demographic behaviour.
I. Definitions and genetic terminology
The name "behaviour geneticist" is used by two distinct groups of researchers. …