One of the most thoroughly documented reaction norms for humans is age at menarche, which has been found to vary by nearly 50 percent across populations, from a low median age of just over 12 years in consistently well-nourished, healthy populations, to up to 18 years in persistently poorly nourished, less healthy ones (Eveleth and Tanner, 1990; Worthman, 1999a).
One might argue that the correlation of environmental quality with age at menarche reflects the impact of environmental insult rather than adaptive biological response, but the case of girls adopted at different ages into radically improved circumstances contradicts this interpretation. Girls from disadvantaged south Indian populations adopted at age 3 or later had an earlier age at menarche (11.1 years) than those adopted at 2 years or less (11.8 years); Proos et al., 1991). Furthermore, recent epidemiological studies of the effects of early environment on systems that drive resource allocation (Barker, 1991, 1997; Clark et al., 1996; Fall et al., 1995) have documented that fetal programming of neuro-endocrine regulation alters developmental and health outcomes across the life span (Adair, 2001; Godfrey, 1998; McDade et al., 2001; Susser and Levin, 1999; Williams and Poulton, 1999).
Secular trends and population variations in timing of reproductive maturation (indexed by menarche for girls) are related to variations in the timing of the onset of puberty in both sexes. Intensive investigation into these variations has identified maternal well-being, infant and child health, nutrition, and psychological well-being as salient ecological correlates. In teleological terms of life history theory (see Figure 10-2), maternal well-being (health, nutrition, and low stress), low juvenile mortality risk (indexed by morbidity), and sustained good nutrition reduce the resources needed for maintenance, with the net effect of increasing productivity.
Greater productivity increases the energy available for growth (in height or weight, fat or muscle) or reproduction and reduces the marginal costs of reproduction. This allows faster growth and accelerated maturation, conducive to earlier puberty. Good conditions from gestation through childhood also signal that the risk or relative cost of reproduction likely will be low (the dashed line in the figure).
At this point, limitations on the applicability of life history theory to intraspecific variations become clear, for life history predicts that adult mortality risk will be associated with earlier onset of the reproductive career (Charnov, 1993). Humans show the opposite pattern, with mortality risk associated with later puberty. As child health and survival improve in a population, child maturation accelerates, as evidenced by increased height for age and earlier age at menarche. Indeed, so close is this link that child