recent years, genetic findings have made it clear that the liability to autism extends much more broadly than the severely handicapping condition that is traditionally diagnosed. Very little is known about the fertility shown by individuals with this broader phenotype.
Conversely, antisocial behavior is associated with a larger average family size and a considerable tendency to start having children in the teenage years (Rutter et al., 1998; Moffitt and the E-Risk Team, 2002).
Second, there have been major trends in fertility patterns in recent years. The average number of children born to each couple has dropped markedly in most industrialized countries, and in many European countries the number is well below 2—and hence below population replacement rates (Hess, 1995). This has been associated with a parallel marked rise in the average age at which individuals have their first child. It might be expected from these figures that there would have been a very sharp fall in the number of births to teenage parents, but at least in the United States and the United Kingdom, the drop has been quite modest. The implication is that there has probably been a change over time in the pattern of births. Another change has been a major increase in the number of multiple births in most industrialized countries, this being a consequence of an increase in the use of various methods of assisted conception (Derom and Bryan, 2000).
Third, in many countries there are marked variations in average completed family size among ethnic groups. For example, in the United Kingdom the average number of children per family is much higher in people of Pakistani or Bangladeshi origin than in “white” families (Modood et al., 1997). There are even more marked differences in the pattern of births. Thus, in the United Kingdom the proportion of children born to one parent of Afro Caribbean origin and one Caucasian parent is very high, whereas the comparable figure is very low for those of Bangladeshi origin (39 versus 1 percent). Similarly, in the United Kingdom there are very marked differences among ethnic groups in the proportion of children reared by a single parent (Modood et al., 1997).
Concerns have been expressed over evidence that in many countries there has been a marked fall in men’s sperm counts (Bostofte et al., 1983; Swan et al., 1997). It is not clear whether this has resulted in reduced male fecundity, but there may have been some effects of that kind. Whether there have been parallel changes in female fecundity is not yet known and the fecundity in both sexes needs to be considered in relation to possible effects on number of births (Joffe, 2000).
Finally, there is much evidence of major changes over time in a range of behaviors likely to influence fertility (Rutter and Smith, 1995). For example, there has been a substantial fall in the age at which children first have sexual intercourse, and there has been a marked fall in the proportion of children born within a legal marriage and a corresponding rise in those