Humans are determinate growers who grow slowly over a protracted period and then exhibit a growth spurt during a relatively late puberty. In puberty, adult height is achieved and energy previously used for growth becomes available for reproduction. Shifts in energy storage through changes in body composition (girls to fat, boys to muscle) attend this transition. After a phase of subfecundity, women enter a period of reproductive activity sustained over roughly two decades. Notably, they possess the unusual feature of programmed cessation of reproductive function—menopause—whereby ovarian potency declines and ceases decades before life ends. Reproductive aging leads to a remarkably ubiquitous average age at last birth of around 40 years (Bongaarts and Potter, 1983). The reproductive careers of men are not so curtailed, although they also show signs of reproductive aging.
Life history theory has excited interest because it applies formal and game theoretical models to comparative data; provides a life span, time-integrated framework; integrates across components of phenotype rather than focusing on specific features; identifies key cost-benefit trade-offs in the design of life history strategies; and thus suggests organic design criteria and evolutionary constraints. It promises to be generalizable, predictive, and hypothesis generating. Formerly, life history analysis was primarily based on species averages for the various life history parameters; variance was not included in formal analysis. However, the range of behavioral decisions used has been expanded to include state-dependent action (condition-, context-, or density-dependent) in models that can evaluate variations within and between individuals (Brommer, 2000). Notable limitations remain. These limitations do not impugn the importance and value of life history theory but should inform its application to fertility behavior.
First, variations within taxa should be distinguished from variations across taxa. Life history theory is based on formal comparative analysis across taxa, yet the goal of the present volume is to address fertility behavior within one taxon, humans. The hierarchy of life history trade-offs derived from analysis encompassing macrotaxonomic variations (between classes or phyla—e.g. fishes versus mammals) does not necessarily generalize directly to variations within species. By definition, different taxa do not share evolutionary history and thus have different design features and life history strategies, so each taxon has a different hierarchy of allocation trade-offs, as allometric analysis confirms. Analysis involving restriction of the phyletic range (within rather than across orders or families) would narrow the sweep of organic design questions and help focus on the relevant variance to partition.