Good news: Homicide has dropped out of the top 15 causes of death in the United States for the first time in 45 years, according to newly released 2010 data from the CDC. Heart disease tops the chart, followed by cancer and lower respiratory diseases. Though the murder rate has traditionally ranked fairly low — peaking at #10 spot in 1989 — it's been a factor since the mid-60s. (As of 2010, it was supplanted on the list by pneumonitis, which happens when food or vomit accidentally go down the windpipe and is typically seen in victims 75 and older.) What's responsible for the country's declining murder rates? Three theories:

1. Fatally abusive relationships are on the decline
"We've taken the home out of homicide," says James Alan Fox, a criminologist from Northeastern University who specializes in murder data. Abusive relationships are ending less frequently in violence, thanks to better outreach for victims and an increasing number of incarcerations for abusive partners.

2. Law enforcement has improved
Several major cities saw a decline in the murder rate in 2010: New York City, Detroit, Michigan, Washington, and Los Angeles — surprising figures, especially considering the economic downturn. "It was long thought that violent crime increased in a troubled economy," says Mark Duell at the Daily Mail, "but criminologists say the recent declines could simply be thanks to better police work." 

3. The population is getting older
The largest segment of the population now falls into the 50+ category. "Younger people — who are most likely to commit or fall victim to murder — are making up a smaller share of the population," says the Associated Press. This supports the CDC's data, as age-related afflictions like Alzheimer's and Parkinson's disease all saw increases. "The risk of homicide declines with age," says Fox, "and the risk of death by disease increases." 

Sources: Associated Press, Daily Mail, NPR