have been achieved, and the implications of this research for healthful adolescent development in the twenty-first century.

Adolescence is a time of metamorphosis. Some changes are biologically —and others socioculturally—determined. The latter are often referred to as the “developmental tasks” of adolescence and are discussed in the next section. From a biological viewpoint, during adolescence, children become adults. They experience puberty, acquire reproductive capabilities, and secondary sexual characteristics, and grow to reach full adult height (Susman, 1997). Hormones are primarily responsible for the biological changes in adolescence (Hopwood et al., 1990; Buchanan et al., 1992). The primary hormones begin to differ by gender at about age 11 (Susman, 1997), with the most dramatic biological changes occurring during early adolescence, between the ages of 12 and 16 (Tanner, 1978). The timing of biological changes differs by gender and racial or ethnic group (Money, 1980; Susman, 1997). On average, females mature 1–2 years ahead of males, and reach their full adult height by age 14 (Marshall and Tanner, 1970); African American females mature even earlier than white females (Kaplowitz and Oberfield, 1999). Interestingly, the age of pubertal onset, as measured by menarche in females, has become younger throughout the past century, most likely due to improved nutritional intake (Wyshak and Frisch, 1982; Hopwood et al., 1990; Herman-Giddens et al., 1997; Kaplowitz and Oberfield, 1999).

Hormonal changes have been “blamed” for their influence on adolescent mood and behavior. Recent provocative research suggests that hormones do influence behavior; the direct effects are small but stable and may be mediated by the social environment (Buchanan et al., 1992; Brooks-Gunn et al., 1994; Susman, 1997). For example, Olweus et al. (1988) found that boys with relatively higher testosterone levels were more likely to become aggressive if they were provoked. Thus, some boys might never exhibit aggressive behavior in a social environment without provocation. Similarly, Udry (1988) found that testosterone levels were a strong predictor of sexual involvement among young adolescent girls. This relationship was attenuated or eliminated by their involvement in sports or having a father in the home. Again, the hormone-behavior relationship existed, but did not manifest in particular environments.

Substantial research has been done on the timing of puberty relative to peers (Silbereisen et al., 1989). Early maturers are differentiated from on-time and late maturers in terms of the development of secondary sexual characteristics, height velocity, and menarche (Silbereisen and Kracke, 1993). Most studies in the United States have found that early maturation among boys is positive for them, with early maturers reporting more confidence, less dependence, and greater popularity than their peers (Nottelmann et al., 1990; Petersen and Taylor, 1980).