The duration of human maturation has been underestimated; an additional 4-6-year pre-adult period of "emerging adulthood," should be included in models of human maturation. It is a period of brain maturation, learning about intimacy and mutual support, intensification of pre-existing friendships, family-oriented socialization, and the attainment of those social skills that are needed for mating and reproduction. We propose that emerging adulthood is a life-history stage that is a foundation of the high reproductive success of human beings. The period of emerging adulthood has an evolutionary context and developmental markers, and we present evidence that supports the idea that emerging adults require protection because they are still learning and maturing. Brain size may be a pacemaker in mammalian life history ( 21 ), and it underlies the remarkable human capacity for learning and communication, but the length of the brain's developmental trajectory was until recently underestimated. It is now clear that brain development does not stop with the completion of puberty when adult brain size is attained. Brain maturation continues beyond adolescence, extending until around age 25 years, and this recently discovered prolongation provides critical support for emerging adulthood as a post-adolescent maturational stage ( 22 ). (Lower) the age range to complete social maturation ( 83 ) and US Bureau of Labor Statistics, 2014. SFO, The subfornical organ; SLF, The superior longitudinal fasciculus; IFO, anterior insula/frontal operculum complex; EA, emerging adulthood; F, female; M, male ( 82 – 84 ). Growth-related Definition of the Transition to Emerging Adulthood To define the transition from adolescence to emerging adulthood, we use the age at which growth velocity returns to prepubertal levels ( Figure 2A ). Takeoff velocity occurs early in gorillas, and despite their greater body mass, female gorillas become sexually mature at a younger age than female chimpanzees ( 44 ). Similar to humans, vervet ( Cercopithecus aethiops ) and rhesus monkeys ( Macaca mulatta ) show a relatively late return to prepubertal growth velocity. Interestingly, this positive correlation between the age at return to prepubertal growth velocity curve and body mass also exists in six small-scale societies described in Walker's Database for Indigenous Cultural Evolution ( http://dice.missouri.edu/ ) ( Figures 2C,D ). Central to the concept of sexual selection is the attainment and optimization of reproductive competence, and the key traits for selection are growth, maturation, and the age at transition to adulthood and sexual reproduction ( 12 ). Figure 3 Comparison of the life-history stages and the newly defined emerging adulthood of Australopithecus afarensis, Homo habilis, H. erectus , and H. sapiens . Human beings and the great apes share similar traits including, to some extent, emerging adulthood. We know relatively little about neurological maturation in non-human primates, but we do know that non-human great apes have a 2-year period of post-menarcheal infertility ( 50 ), extended in human foragers to 3 years ( 51 ). Low reproductive success among young females is a general primate phenomenon ( 52 ). Male preference for fully developed adult females has been described in 15 primate species ( 52 ). Goodall reported that after menarche, which usually occurs at age 10 years, female chimpanzees average 19 full-size cycles before becoming pregnant for the first time at age 12 years ( 53 ). Late reproduction should decrease fitness ( 60 ), but several tradeoffs could influence the prolonged period of emerging adulthood in human life-history strategies: reproducing at an earlier or later age; reproducing at a young age or continuing to grow and develop; and being an adult parent with a large parental investment in each offspring of a small family or a young parent with a small parental investment in each offspring of a large family. The Charnov model predicts that a long life span will be associated with slow development, iteroparity (repeated reproduction), a single offspring, and long parental care ( 59 ). The hunting performance and ability of Tsimane foragers is another example of a skill whose acquisition depends more on age than strength. Thus, the evidence from foraging societies and the conditions to which humans became adapted during our evolution show that neither reproductive behaviors (i.e., parenting and the ability to manage the relationship with a spouse) nor subsistence skills are mastered by the end of adolescence. Even in societies where children forage from an age as young as four, their efficiency as young adults remains lower than that of their mothers ( 110 ). Recent imaging studies have shown that brain development continues throughout emerging adulthood; maturation of the neocortical association areas, notably the frontal lobes, extends into the mid-twenties, and is still incomplete long after the end of puberty and linear body growth. There is now abundant evidence that the frequency of behavioral disturbances of adolescence, such as unplanned sexual activity, risk-taking, impulsivity, depression, and delinquency, declines after adolescence despite persistent high levels of gonadal hormones. The most likely explanation for the transient nature of these behavioral disturbances of adolescence is continuing myelination of the frontal cortex and other brain regions that are involved in the executive control of impulses and emotions. Adolescence is often delayed in foraging societies, resembling our human environments of evolutionary adaptedness. Since the women in these societies have late menarche and are subsequently subfertile, the age of these young women at the time of first birth is 19 years and their husbands are generally several years older. These young parents are strongly supported by older family members, who supply needed food and advice.

The rate at which the brain develops from conception to adulthood, and how that maturational process relates to brain's product, behavior, has been a major topic for the neurosciences with implications to legal practice, most dramatically in the case of the death penalty. Based partly on neuroscience evidence for protracted maturation of executive brain systems, the US Supreme Court ruled in 2005 that capital punishment is unconstitutional for crimes committed while under the age of 18. Much more data have since been acquired on multiple indices of brain development and behavior, and a question now facing the courts is whether the cutoff age of 18 has a scientific basis or, from the developmental neuroscience evidence, it is arbitrary and in need for re-evaluation. Here I present methods used by neuroscientists to measure parameters of brain development related to complex behavior, and summarize the major findings to date. Since 2005, there have been several large-scale studies revealing multimodal brain parameters related to major domains of behavior. These studies converge to show protracted development of brain tissue and its connectivity, as well as physiologic parameters of resting-state and task-related activity, which underlie the ability of the brain to process and integrate information. The developmental course of these differences does not reach its apex at 18, indeed it continues into the early 20s. I conclude that this 18 and older population, probably up to around age 23, is therefore just as vulnerable to the effects on behavior (especially behavioral control) of the lack of maturation of these brain structures and functions as are 17-year-olds.