also lead to plasticity, in which individual organisms can shift along the spectrum of r-selected vs. K-selected life histories to suit the environment. Human life history In studying humans, life history theory is used in many ways, including in biology, psychology, economics, anthropology, and other fields. For humans, life history strategies include all the usual factors—trade-offs, constraints, reproductive effort, etc.—but also includes a culture factor that allows them to solve problems through cultural means in addition to through adaptation. Humans also have unique traits that make them stand out from other organisms, such as a large brain, later maturity and age of first reproduction, a long lifespan, and a high level of reproduction, often supported by fathers and older (post-menopausal) relatives. There are a variety of possible explanations for these unique traits. For example, a long juvenile period may have been adapted to support a period of learning the skills needed for successful hunting and foraging. This period of learning may also explain the longer lifespan, as a longer amount of time over which to use those skills makes the period needed to acquire them worth it. Cooperative breeding and the grandmothering hypothesis have been proposed as the reasons that humans continue to live for many years after they are no longer capable of reproducing. The large brain allows for a greater learning capacity, and the ability to engage in new behaviors and create new things. The change in brain size may have been the result of a dietary shift—towards higher quality and difficult to obtain food sources—or may have been driven by the social requirements of group living, which promoted sharing and provisioning. Recent authors, such as Kaplan, argue that both aspects are probably important. Research has also indicated that humans may pursue different reproductive strategies. Tools used mathematical modeling quantitative genetics artificial selection demography optimality modeling mechanistic approach Malthusian parameter Perspectives 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 and there is a large body of scientific literature from studies in experimental animal models, and naturalistic studies among many organisms. Criticism The claim that long periods of helplessness in young would select for more parenting effort in protecting the young at the same time as high levels of predation would select for