Maine is the most peaceful state in America and Louisiana the least, according to rankings by an Australian think tank called the Institute for Economics and Peace.

The rankings are based on the prevalence of violent crimes, homicides, police employees, size of the prison population and small arms availability.

Overall, 2011 was the most peaceful year the United States has experienced in 20 years. Homicides and violent crimes both dropped by more than 3 percent last year, while the murder rate has plummeted a staggering 50 percent since 1991, when the survey first started. However, prison violence—which is not counted in the report—has risen dramatically as the number of people behind bars has also grown. (The United States has a higher percentage of its population incarcerated than any other nation.) Almost half of all forcible rapes occur in prison.

Maine, Vermont, New Hampshire, Minnesota and Utah were the top five most peaceful states. Louisiana was the least peaceful, followed by Tennessee, Nevada, Florida and Arizona.

The Cambridge-Newton-Framingham area of Massachusetts ranked as the most peaceful metro area, while Detroit-Livonia-Dearborn was ranked the least peaceful.

The report found that out of dozens of environmental, economic and health factors, the percentage of children living in two-parent families was the factor most correlated to peacefulness. The more children living in single-parent families, the less peaceful a state was. Poverty was also a strong indicator. None of the top 10 most peaceful states had a poverty ranking that cracked 10 percent.

The report estimates that the average taxpayer pays $3,257 per year on violence and violence containment, while the total economic cost is $460 billion. The cost of violence includes the health costs of victims of violent crimes and homicides, incarceration costs and other factors.

New York, California and Michigan showed the biggest improvement in peacefulness from 1991, while Montana, North Dakota and South Dakota showed the biggest swing away from peacefulness over the same period.

Story continues

More popular Yahoo! News stories:

• Arizona's illegal-immigration law heads to Supreme Court. Will justices strike it down?

• Romney returning to New Hampshire to mark 'semiofficial end' of GOP primary

• Orrin Hatch narrowly forced into Utah Senate primary