Adolescence Duration and Characteristics
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Adolescence is a stage in human life history covering the five to ten years after the onset of puberty until the onset of adulthood. The adolescent phase is characterized by a growth spurt in height and weight, the development of secondary sexual characteristics, sociosexual maturation, and intensification of interest and practice adult social, economic, and sexual activities. An evolutionary derived characteristic of human adolescent girls is that they experience menarche (first menstruation) about 9 months after the peak velocity of the adolescent growth spurt. Girls then have 1 to 3 years of adolescent sterility (absent or infrequent ovulatory cycles). Human adolescent boys are fecund before the peak velocity of the spurt, but delay adult appearance in terms of height and muscularity until abut age 18 years. Other primate species, especially the apes, do not have a global skeletal growth spurt and females have only about 1 year of sub-fecundity following the initiation of estrus. Post-pubertal chimpanzee and gorilla males lack the global skeletal growth spurt, but do have spurts in dento-facial growth due to the eruption of projecting permenant canine teeth and in body mass due to rapid growth of muscle mass. Chimapnzee males share with human boys the slow development of post-pubertal adult socio-sexual morphology. Human adolescence ends with the cessation of skeletal growth in length (the closing of the epiphyses of the long bones), the completion of dental development (eruption of the third molar, if it is present), and sexual maturation (measured for women as the age at first reproduction). On a worldwide basis, including living and historical societies, the age of onset of adulthood averages 19 years for women and 21 to 25 years for men. Both natural selection, for reduced infant and maternal mortality, and sexual selection for enhanced mating opportunities are responsible for the evolutionary selection for the adolescent life history stage. Fossil evidence indicates that an adolescent life history stage may have evolved with the appearance of Homo sapiens (~125,000 BP), or possibly with H. antecessor (~780,000 BP).