Adolescence, the transitional stage of development between childhood and adulthood, represents the period of time during which a person experiences a variety of biological changes and encounters a number of emotional issues. The ages which are considered to be part of adolescence vary by culture, and ranges from preteens to nineteen years. According to the World Health Organization (WHO), adolescence covers the period of life between 10 and 20 years of age. Adolescence is often divided by psychologists into three distinct phases: early, mid and late adolescence.
Adolescence can be a specifically turbulent as well as a dynamic period of one's life. It has been identified as a period in which young people develop abstract thinking abilities, become more aware of their sexuality, develop a clearer sense of psychological identity, and increase their independence from parents. G. Stanley Hall denoted this period as one of "Storm and Stress" and, according to him, conflict at this developmental stage is normal and not unusual. Margaret Mead, on the other hand, attributed the behavior of adolescents to their culture and upbringing.
Several developmental stage models have placed adolescence in a period of human development. Sigmund Freud saw it as the "genital phase" of psychosexual development, where the child recaptures the sexual awareness of infancy. Jean Piaget focused on cognitive development, seeing adolescence as the "formal operative stage" where the young person develops the ability to think abstractly and draw conclusions from the information available. Erik Erikson’s theory of psychosocial development identified the identity crisis as central to the notion of adolescence.
Adolescent psychology addresses the issues associated with adolescence, such as whether or not the aforementioned "storm and stress" is a normal part of this period. The American Psychological Association has a separate division dedicated to adolescence, and the psychologists specializing in this topic attempt to answer questions dealing with the age group. One issue in adolescent psychology discusses whether adolescence is in fact a discrete developmental period, a point along a continuum of human development, or a social construction.
Early adolescence is a stage at which the peer group becomes increasingly important, with conformity to peers peaking at 11-13 years (Costanzo and Shaw 1966). 90% of adolescents identify themselves with a peer group (Palmonari, 1989). According to Judith Rich Harris’s theory of group socialization, children and adolescents are shaped more by their peers than their parents (Harris 1997). Peers can encourage both pro-social behavior, which peaks at 11-12 years, or anti-social behavior, which peaks at 14-15 years (Bendt, 1979). Adolescents are less likely to feel depressed or anxious if the peer group provides emotional support (Buhrmester, 1992).
Non-human mammals also exhibit changes in social attitude during adolescence. Adolescent rodents have also been observed spending more of their time with rodents of similar age. Conflicts between adolescents and parents have been noted in other primates, and overall increases in aggressiveness have been observed during this time period. Despite this, social bonding between adolescents and adults tends to improve due to reconciliatory behavior. Allomaternal behavior increases among females in several species, including humans, nonhuman primates, and rodents. However, males tend to exhibit less interest in infants during adolescence.
Although most adolescents are psychologically healthy, they can (like adults) exhibit signs of mental illness. Late adolescence and early adulthood are peak years for the onset of schizophrenia. Mood disorders such as clinical depression and bipolar disorder can initially show in adolescence. Girls aged between 15 and 19 make up 40% of anorexia nervosa cases.