Music lyrics have undergone dramatic changes since the introduction of rock music more than 40 years ago. This is an issue of vital interest and concern for parents and pediatricians. During the past four decades, rock music lyrics have become increasingly explicit—particularly with reference to sex, drugs, and violence. Recently, heavy metal and "gangsta rap" music lyrics have elicited the greatest concern. In some cases lyrics communicate potentially harmful health messages. Such lyrics are of special concern in today's environment, which poses unprecedented threats to the health and well-being of adolescents. Pregnancy, drug use, acquired immunodeficiency syndrome (and other sexually transmitted diseases), injuries, homicide, and suicide have all become part of the landscape of everyday life for many American teens. At the same time, music is important to teenagers' identity and helps them define important social and subcultural boundaries. The results of one survey of 2760 14-to 16-year-olds in 10 different southeastern cities showed that that they listened to music an average of 40 hours per week. One Swedish study found that adolescents who developed an early interest in rock music were more likely to be influenced by their peers and less influenced by their parents than older adolescents. To date, no studies have documented a cause-and-effect relationship between sexually explicit or violent lyrics and adverse behavioral effects. A possible explanation for this lack of finding is that teenagers often do not know the lyrics or fully comprehend their meaning. For example, in one study only 30% of teenagers knew the lyrics to their favorite songs, and their comprehension varied greatly.