Last January in Freehold Township, N.J., a car driven by a 17-year-old high school student, with two fellow students as passengers, passed a car being driven by another teenager at 70 miles an hour in a 50-mile zone. The passing vehicle crashed into an oncoming van. Three boys and the 68-year-old van driver were killed.

It is an all too familiar tale, prompting parents and school officials alike to wonder why risky behavior is so common among teenagers and what might be done to curtail it. Is it that teenagers think that they are immortal or invulnerable, immune to the hazards adults see so clearly? Or do they not appreciate the risks involved and need repeated reminders of the dangers inherent in activities like driving too fast, driving drunk, having unprotected sex, experimenting with drugs, binge drinking, jumping into unknown waters, you name it?

None of the above, says Valerie F. Reyna, professor of human development and psychology at the New York State College of Human Ecology at Cornell. The facts are quite the opposite. Scientific studies have shown that adolescents are very well aware of their vulnerability and that they actually overestimate their risk of suffering negative effects from activities like drinking and unprotected sex.

For example, a study by researchers at the University of California, San Francisco, found that teenagers were more likely than adults to overestimate risks for every outcome studied, from low-probability events like contracting H.I.V. to higher-probability ones like acquiring more common sexually transmitted diseases or becoming pregnant from a single act of unprotected sex.