ABSTRACT
This chapter presents analyses designed to describe phenotypic correlations between life history traits and then measure trade-offs between traits. Modern life history theory is based on the assumption that limited time, energy, and resources are available for expenditure. Reaction norms are genetically evolved response patterns that produce phenotypic variation keyed to relevant environmental variation. They result in adaptive life history variation in which the phenotypic variation itself has no genetic basis but is instead induced by key ecological variables. Ache demographic data enable us to search for evidence of the cost of reproduction in a noncontracepting human population living under conditions that are likely to be similar to those under which fertility trade-off mechanisms evolved in humans. Men and women in modern industrial societies are opting for lower fertility still, and no available model can yet explain phenomena. The results indicate a relationship between age at first birth and adult fertility.
