The majority of adolescents in the United States have had sexual intercourse by the time they finish high school.  Data from a nationwide survey of high school students show that more than one-third of adolescents have had intercourse by the ninth grade, and nearly two-thirds by twelfth grade. Rates of engaging in oral sex are much higher than for sexual intercourse, with 20 to 30% of high school students and 18% of teens between the ages of 12 and 15 reporting that they had engaged in oral sex at least once (Halpern-Felsher, Cornell, Kropp, & Tschann, 2005; McKay, 2004).

Why do some adolescents become sexually active at an early age while others wait? The reasons vary, but several tendencies exist. In general, adolescents who engage in earlier sexual activity are more likely to live in poverty, perform poorly in school, achieve lower intelligence-test scores, lack clear educational goals, live in single-parent homes, report poor parental communication and lack of parental support, be more impulsive, and use alcohol and other drugs and engage in binge drinking. The top reasons teens give for being sexually active are peer pressure, curiosity, and the idea that "everyone does it." Girls also cite pressure from boys as a top reason, and girls who mature earlier and look older are more likely to be sexually active than those who mature later (Dunn, Bartee, & Perko, 2003; Katchadourian, 1990; Raffaelli & Crockett, 2003; Savin-Williams & Diamond, 2004). Among teens who had abstained from sex, 65% told researchers they abstained because they feared sexually transmitted diseases (or STDs, diseases that are transmitted mostly through sexual contact); 62% feared pregnancy, 50% feared parents; and 29% feared they would lose their reputations among friends (Katchadourian, 1990).

Lack of parental supervision is an important factor in teens' sexual activity. One study reported that higher levels of parental supervision were significantly linked to lower levels of sexual activity, smaller number of sexual partners, and lower incidence of STDs, especially for boys (Cohen, Farley, Taylor, Martin, & Schuster, 2002). The majority of high school students said the most frequent place they had sex was in their own homes (74% of boys and 87% of girls), and more than half said they had sex at home after school—when their parents were not home. Supervision of adolescents is a difficult issue for parents, especially for those parents who work outside of the home. Unfortunately, there are fewer supervised after-school activities available to middle and high school youths than there are for younger kids. And at some point, parents do need to "loosen the reins" and allow teens more unsupervised time. The questions are how much, how soon, and under what conditions—difficult questions to answer.

Not surprisingly, many adults are concerned about adolescents' consistent and correct use of contraception, methods used to prevent pregnancy and sexually transmitted diseases. More adolescents report using contraception (particularly condoms and birth control pills) now than in past years. But teens are still notoriously inconsistent in this regard. For example, 45% of adolescent girls under age 16 in a recent survey did not use any contraception at first intercourse (Kourtis et al. 2006). The best predictor of consistent contraceptive use is age-though not perfect, older teens are more reliable than younger ones. Family factors are also important. Teens who have good relationships with their families are less likely to be sexually active, and they're more likely to consistently use contraceptives if they are sexually active (Abma, Martinez, Mosher, & Dawson, 2004; Breheny & Stephens, 2004; Mosher, Martinez, Chandra, Abma, & Willson, 2004; Santelli, Lowry, Brener, & Robin, 2000).

Why don't teens consistently use contraception? Many teens simply are not mature enough to admit to themselves that they are sexually active or that they plan to be. Consistent contraceptive use, especially hormonal methods like birth control pills, requires that adolescents recognize they are sexually active and plan for sex far in advance of the act itself—something that is incompatible with many teens' views of sexual activity as being spontaneous. Some teens do not use contraceptives because they're afraid their parents will discover that they are sexually active. Others say they do not know where to get contraceptives or that they aren't easily available. Surprisingly, despite the years of sex education that most teens have, some still have misunderstandings about sex and pregnancy or hold misconceptions about the need for contraceptives and how to use them. For example, some teens believe that if they do not have sex frequently, they do not need contraceptives, or that taking a single birth control pill shortly before or after having sex will protect them. And some teens simply don't believe that things like pregnancy or STDs will happen to them. The belief that one is special and invincible is called the personal fable, and is related to cognitive development during adolescence (see Chapter 5). Finally, for some adolescents, lack of contraceptive use is part of a broader pattern of risk-taking behaviors and impulsivity (Averett, Rees, & Argus, 2002; Iuliano, Speizer, Santelli, & Kendall, 2006; Kaplan, 2004; Miller & Moore, 1990).

Almost all public school systems provide sex education programs. Churches and community organizations such as Planned Parenthood, the YMCA, Boy Scouts, and Girl Scouts also offer these programs. Recent studies find that students' knowledge of sex and contraception is increased by participating in sex education programs, though they don't always participate in these programs before they become sexually active (Kirby, 2002; Lindberg, Ku, & Sonenstein, 2000). However, a surprising number of teens have little knowledge of what STDs are, how they are spread, and how to prevent them. For example, only one third of teens in one study knew that chlamydia is an STD. Another found that many teens in their sample believed that condoms cause STDs (Downs, deBruin, Murray, & Fischhoff, 2006; Garside, Ayres, & Owen, 2001; Halpern-Felsher et al., 2005; Hamilton, Ventura, Martin, & Sutton, 2005; Lagerberg, 2004; McKay, 2004).

Almost everyone agrees that it is essential to provide sex education in schools, but there is heated debate about what to teach in sex-ed classes. Read about the debate between abstinence-only and comprehensive sex-ed programs in the Social Policy Perspective on page 401, "The Sex Education Debate."