Timing of pubertal maturation affects adolescent adjustment in an opposite way for girls than for boys, and the effects of maturational timing involve a complex blend of biological, social, and cultural factors. The arrival of puberty is accompanied by new health concerns. For some teenagers, the cultural ideal of thinness combines with family and psychological problems to produce the serious eating disorders of anorexia nervosa and bulimia nervosa. Teenage sexual activity in the United States has increased over several decades but recently declined slightly. Sexual orientation is affected strongly by heredity but also by a variety of biological and environmental combinations.
Adolescents have the highest rate of sexually transmitted disease of any age group.
Sexual activity is accompanied by high rates of adolescent pregnancy and parenthood. Although most teenagers engage in some experimentation with alcohol and drugs, a worrisome minority make the transition from use to abuse. Unintentional injuries increase in adolescence, largely due to motor vehicle collisions-the leading killer of American teenagers. Firearm injuries and deaths are also high in the United States. During adolescence, both sexes improve in gross motor performance, although boys show much larger gains than girls.