Homicide rates among teenagers and young adults have dropped to the lowest level in 30 years, say the Centers for Disease Control and Prevention. That’s good news, reports NPR, but it still means 4,800 young people under age 25 were murdered in 2010. Teens and young adults remain more likely to be killed than older adults, and homicide is a leading cause of death in the young, behind vehicle accidents. Homicide rates have dropped steadily since an uptick in the early 1990s, says CDC in Morbidity and Mortality Weekly Report. The rate for ages 10 to 24 was 7.5 per 100,000 in 2010, compared to 15.9 in 1993. That mirrors a long decline in crime overall. There are plenty of theories for why, including better policing, more incarceration, and the economic boom of the 1990s. No theory has been proved. “In short, we don’t know,” says Dr. Matthew Miller of the Harvard School of Public Health. “The usual suspects don’t seem to explain it.”