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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.
Keywords: life history, adolescence, human evolution, hominin, comparative development, brain development
Growing evidence suggests that an individual at the end of adolescence cannot be considered to be an adult when using physical, physiological, intellectual, social, emotional, and behavioral measures. When adolescents in developed societies mature and achieve adult body size, their behavior often remains immature. Specialists in adolescent medicine have recognized this incongruity, and have redefined adolescence to include young adults up to age 24 years, of whom many have not yet assumed adult roles (1, 2). Reproduction in contemporary forager societies also begins several years after adolescence and post-adolescent individuals are often limited in their gathering and/or hunting skills (3–5). Compared to other mammals, primates produce few offspring. Humans have an even slower growth rate than that of non-human primates of comparable size, but human growth may be even more prolonged than is generally realized.
Arnett proposed emerging adulthood as a phase of life between adolescence and full-fledged adulthood, with distinctive demographic, social, and subjective psychological features (6, 7). This life- history stage applies to individuals aged between 18 and 25 years, the period during which they become more economically independent by training and/or education. Previously, the psychodynamic theorist Erik Erikson identified a stage that he called a prolonged adolescence or psychosocial moratorium in young people in developed societies (8, 9). Much more recently, Hopwood and colleagues explored genetic and environmental influences on personality development during the transition to adulthood in same-sex male and female monozygotic and dizygotic twins assessed in late adolescence (approximately age 17 years), emerging adulthood (~24 years), and young adulthood (~29 years) (10). Their genetically-informed results support a life-course perspective on personality development during the transition to adulthood. In addition, the United Nations has identified youth, defined as the period from 15 to 24 years of age, as a period of vulnerability worldwide and has made it a priority for multiple interventions (11).
Here, we use an evolutionary approach in order to understand emerging adulthood, arguing that it is not just a sociological transition period but a biological life-history phase. Trait variability, whether it is molecular, cellular, physiological, morphological, or behavioral, is the leading edge of evolution. Together with genetic evolution, plasticity in developmental programming has evolved to provide the organism with traits that can secure its survival and reproductive success (12). Life-history theory is a powerful tool for understanding child growth and development from an evolutionary perspective (2, 13, 14). We provide evidence that emerging adulthood exists in some other mammals, which implies genetic evolution, and we discuss emerging adulthood in foraging as well as developed societies, which implies the occurrence of adaptive plasticity and cultural influences. We propose that genetic and cultural evolution have interacted to produce the emerging adulthood stage in human life history.
Determining the exact time of transitions between life-history stages is challenging (13). Saltations (growth spurts) and transitions occur during human growth (15, 16), and stages have a central place in evolutionary life-history theory, but the turning points are theoretical constructions in which some aspects of a transition are highlighted.
Puberty produces an endocrine transformation with striking somatic and behavioral changes, especially in body image, sex identity, aggression, and impulsivity. To define a maturational stage between adolescence and adulthood, we need first to define the end of adolescence. During this transition, growth velocity decelerates, blood and tissue hormone levels increase, aggression becomes less overt, and learning and maturation mitigate hormonal impact.
Using maturational measures avoids the pitfalls of defining emerging adulthood according to chronological age. For example, the Tanner scale of adolescent development is based on external primary and secondary sex characteristics. Tanner stage V recognizes the conclusion of puberty in boys when the testicular volume is >20 ml and the length of the penis is >14 cm (17) and in girls when the breast reaches final adult size and the areola returns to the contour of the surrounding breast with a projecting central papilla (18).
Here, we define the transition between adolescence and emerging adulthood as occurring when growth returns to its prepubertal trajectory and the boy or girl is at Tanner stage IV (19). Boys at this stage have a testicular volume between 12 and 20 ml, their scrotal skin darkens, and the length of their penis is ~10 cm. Girls at this stage have experienced menarche, their breasts are of adult size and elevated, and the areola and papilla form a secondary mound which projects from the contour of the surrounding breast. Body composition continues to change during emerging adulthood, in terms of relative fat mass, lean body mass, and total body bone mineral content and bone mineral density increase (20), but the most important maturational changes after adolescence, even if defined as the end of Tanner Stage 4, are in the brain.
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). Compared to other primates, human newborns are neurologically and behaviorally altricial because many aspects of brain development are protracted, including that of the prefrontal cortex (23). The cortical architectural units or minicolumns in the prefrontal cortex of humans are wider than those of the great apes, an increase that occurs after puberty in humans, but not in chimpanzees (24). In chimpanzees, but not in humans, myelination becomes complete at about the time of sexual maturity (25). Interestingly, human brain regions with protracted development are the same that have undergone the greatest degree of volumetric enlargement in primate evolution (26).
In a large-scale longitudinal pediatric neuroimaging study, brain maturation was found to continue after adolescence: post-adolescent increases in white matter are linear while the changes in the cortical gray matter are non-linear. Cortical white matter in particular continues to increase into the mid-twenties, which is likely related to the efficiency and speed of cortical connectivity (27, 28). In another study, Sowell and her colleagues spatially and temporally mapped brain maturation in North American adolescents (age 12–16 years) and young adults (age 23–30 years) using a whole-brain, voxel-by-voxel statistical analysis of high-resolution structural magnetic resonance images (29). They found that the pattern of brain maturation during these years was distinct from earlier development and was localized to large regions of the dorsal, medial, and orbital frontal cortex and lenticular nuclei. They also reported relatively little change at other brain locations. They concluded that cognitive function improves throughout adolescence, and this improvement is associated with parallel post-adolescent reductions in gray matter density (as white matter increases) in frontal and striatal regions. It has been argued that such brain changes should mitigate the guilt of adolescent delinquents who have not yet gone through them (30–32).
Asato and colleagues also investigated white matter maturation during adolescence using diffusion tensor imaging and reported that (a) pubertal hormones influence white matter development and maturation and (b) white matter connectivity and the executive control of behavior is still immature in adolescence (33). Jolles and colleagues investigated the association between whole-brain functional connectivity and cognitive and emotional functions in children (11–13 years) and young adults (19–25 years) (34). Although they found similar patterns of functional connectivity in children and young adults, there were differences in the size of the functionally connected regions and the strength of functional connectivity. Thus, functional connectivity continues to change during and after adolescence, and these developmental differences in functional connectivity patterns were associated with higher cognitive or emotional functions and basic visual and sensorimotor functions.
In another study comparing social and emotional functioning of children, adolescents, and young adults, by analyzing the age-dependent development of five functionally distinct cingulate-based intrinsic connectivity networks (ICNs), Kelly and colleagues provide additional evidence that brain maturation extends beyond adolescence into young adulthood (35). They found that the pattern of correlation with voxels proximal to the seed region of interest was age-dependent: the pattern was diffuse in children (mean age 10.6 years), was less diffuse in adolescents (mean age 15.4 years), and showed signs of becoming focal in young adults (mean age 22.4 years). Also, the greatest development occurred in those ICNs associated with social and emotional functions. Finally, in their study of the brains of 103 healthy subjects aged 5–32 years using diffusion tensor tractography, Lebel and Beaulieu provide further evidence that brain maturation continues from childhood into adulthood (36). Association tracts show within-subject maturation of measures indicative of myelination and axon density.
Collectively, these studies provide strong evidence that brain development and maturation continue in young adulthood; the idea that brain maturation is finalized during adolescence is no longer tenable. Psychologically, emerging adulthood is a stage when an individual's cognitive abilities increase to reach their peak in their fourth decade and possibly beyond (37). Schaie and colleagues included 13 measures of crystallized abilities influenced by schooling and experience. The critical abilities from this perspective are those that enable the learning of new things, that is, working memory and fluid intelligence; these, as well as processing speed (38), peak in the mid 20s.
Emerging adulthood is also a social stage: it is a period of learning about intimacy and mutual support, intensification of pre-existing friendships, family-oriented socialization, political awareness, developing new relationships, and the attainment of biosocial skills that are needed for successful mating and reproduction. Finally, it is also a stage of understanding self-concepts and ideal concepts, emphasized interpersonal reactivity and obligation, self-expressiveness, and contempt toward particular ideologies (39). The attainment of these cognitive, emotional, and social abilities is the result of a complex interplay of maturation and interaction with the environment, but it is now possible to say that at least in the earlier years of emerging adulthood, they are correlated with and possibly caused by brain maturation. There is also evidence that brain size growth continues into the third decade in some individuals. In these individuals, hypothalamic maturation, puberty, and the resultant hormonal surges are dissociated from and even precede development and maturity of frontal cortex (40, 41).
Emerging adulthood is associated with other physiological changes, such as bone mineral accretion, the completion of growth, and [frequently] first reproduction. Hence, emerging adulthood begins as a physiological, but most importantly a neural transformation in which behavioral and social functions interact, with consequences for impulse control in domains that have put the individual at risk during puberty. We will argue that this life-history phase has unfolded throughout hominin evolution. In Figure 1 we show the timeline of maturation of the main physical, behavioral and social traits.