Know what your kids are listening to when they’re blocking you out with their iPod earbuds firmly in place? If they are listening to popular music, chances are high that they are hearing references to substance use.

According to new research presented at the American Public Health Association’s Annual Meeting & Exposition in Washington, D.C., 33 percent of the most popular songs of 2005 portrayed substance use. The study, in which researchers analyzed 279 of the year’s most popular songs according to Billboard magazine, also found that allusions to substance use varied widely by genre.

Rap music led the way with 77 percent of songs referring to substance use, followed by country at 37 percent and R&B/hip-hop at 20 percent. Rock and pop were on the lower end of the spectrum at 14 percent and 9 percent, respectively.

Alcohol and marijuana were the substances most frequently portrayed. Substance use was commonly associated with partying, sex, violence and/or humor, and was most often motivated by peer/social pressure, sex, and/or money (for instance, through trafficking). The majority of songs with substance use portrayed more positive than negative consequences of use.

“Previous research has shown that exposure to substance use messages in media is linked to actual substance use in adolescents,” said Brian A. Primack, MD, EdM, lead researcher on the study. “That is why we need to be aware of exposures such as these, especially when they are associated with highly positive consequences and associations.”