Dating back to Muller (49), considerable thought has been given to the potential for a cumulative buildup of the deleterious-mutation load in the human population (2, 3, 50, 51). The motivation for this concern is the enormous change in the selective environment that human behavior has induced during approximately the past century. Innovations spawned by agriculture, architecture, industrialization, and most notably a sophisticated health care industry have led to a dramatic relaxation in selection against mildly delete- rious mutations, and modern medical intervention is increasingly successful in ensuring a productive lifespan even in individuals carrying mutations with major morphological, metabolic, and behavioral defects. The statistics are impressive. For example, fetal mortality has declined by approximately 99% in England since the 1500s (52), and just since 1975, the mortality rate per diagnosed cancer has declined by approximately 20% in the United States population (53). Because most complex traits in humans have very high heritabilities (54), the concern then is that unique aspects of human culture, religion, and other social interactions with well intentioned short-term benefits will eventually lead to the long-term genetic deterioration of the human gene pool.
