like shooting at a moving target, with researchers designating “adolescent” groups of different ages and levels of development. Furthermore, from the mid-19th through the 20th century, an earlier average age of menarche has been observed in the western world (Falkner and Tanner, 1986; Tanner, 1990). The educational process is more prolonged and individuals are tending to wait longer before starting their careers, getting married, and having children (Dahl, 2004). Thus, the length of adolescence is not fixed (and has been lengthening) and while the period correlates with many biological developmental processes, it is partially defined according to psychosocial and behavioral criteria. With these caveats in mind, the literature reviewed here has primarily defined adolescence in humans as the second decade of life, in monkeys as age two to four years, and in rodents as week four to week six or seven.
