Perhaps the best evidence for such a capacity to convey average, rather than transient, ecological information, comes from studies of the effects of a mother’s nutrition on the birth weight of her baby.60 Studies generally find that birth weights tend to be lighter in populations in which nutrition has been marginal for multi generations. Despite this evidence for environmental influence on fetal growth rate and birth size supmenting pregnant women generally has minimal effects on the birth weight of offspring. Thus, it appears that long-term history in an environment may be an important influence on the resources transferred in support of offspring growth, but that fluctuations in intake during pregnancy itself, reflected for instance in dietary supmentation, have comparably modes effects. This phenotypic inertia—reflecting the lingering biological but non-genetic effects of the mother’s average experiences in the past—could allow the fetus to track those dynamic features of environments that are relatively stable on the timescale of decades or several generations (see Refs 60 and 68). This could allow adjustment to environmental changes that are too rapid to result In modifications in gene frequencies via natural selection, which requires many generations, but that are too chronic to be buffered efficiently by reversible homeostatic processes. In this way, the mother’s body could pass biological “memories”, reflecting her own lifetime of experiences in the local environment, to her developing offspring allowing developmental adjustments to be made in anticipation of conditions likely to be experienced locally.
IMPLICAIONS OF DEVELOPMENTAL PROGRAMMIING FOR HUMAN HEALTH DISPARTIES
The preceding sections have surveyed some of the evidence that early life stressors can have health effects that linger into adulthood and in some instances may even transcend the present generation to be passed on to offspring. These findings hold promise to help explain why health and disease tend to relate strongly to environmental, social and economic conditions both within and between populations. There is growing evidence that early environment-triggered developmental plasticity can help to clarify two broad problems in public health: disease transitions in populations experiencing rapid cultural, nutritional or lifestyle change, and health disparities within populations marked by chronic inequality and social stratification related to class, ethnicity or social “race”. The discussion ends with a brief review of the evidence that developmental processes contribute to each of these public health issues.