Life-history theory

Life history theory is a method of analysis in animal and human biology, psychology, and especially evolutionary sociobiology which postulates that many of the physiological traits and behaviors of individuals may be best understood in relation to the key maturational and reproductive characteristics that define the life course.

Examples of these characteristics include:
 * Age at weaning
 * Age of sexual maturity or puberty
 * Adult body size
 * Time to first sexual activity or mating
 * Time to first reproduction
 * Duration of gestation
 * Litter size
 * Interbirth interval

Variations of these characteristics reflect differing allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions, especially growth, body maintenance, and reproduction. For an individual, the resources in a particular environment are finite. Time, effort, and energy used for one purpose diminishes the time effort, and energy available for another. For example, resources spent growing to a larger body size cannot be spent increasing the number of offspring.

Thus allocation of resources involves trade-offs. These trade-offs and strategies can be compared between species. Two of the most well-known trade-offs involve number of offspring (few or many) and timing of reproduction (accelerated maturation and reproduction versus delayed, allowing for larger size and more complex social supports). The extremes at the species level of these fundamental dimensions of reproduction were recognized long before life history theory, and are traditionally termed r/K selection theory. An r-selection strategy is the production of a large number of offspring (of whom only a minority may survive) as early in life as possible. The K-selection strategy is to produce a smaller number of "fitter" offspring with higher survival chances.

According to life history theory the individuals of a species are able to make limited shifts in reproductive strategies in response to the prevailing environments. Depending on abundance of resources and probable individual longevity, individuals consciously or unconsciously shift their reproductive strategy in one direction or the other to take advantage of available resources or to compensate for resource shortage or uncertainty.

Life histories of individual human beings may be analyzed from the same perspectives, and the theory provides some explanatory and predictive power when applied to groups of people in different life situations. For example, it predicts that in a stressful environment, or one with uncertain resources, sexual development, mating, and pregnancy may be accelerated, with acceptance of higher risks and production of more children, while security and high resource availability tend to favor slower maturation, later mating, and fewer offspring.

Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility. A number of statistical predictions have been confirmed by social data, though not always reproducibly. The implications for social policy have been hotly debated because statistical associations are not always causal, and a preferred interpretation may be a more minor factor than another unpalatable relationship.