The Supreme Court’s landmark ruling on gun ownership last week focused on citizens’ ability to defend themselves from intruders in their homes. But research shows that surprisingly often, gun owners use the weapons on themselves.

Suicides accounted for 55 percent of the nation’s nearly 31,000 firearm deaths in 2005, the most recent year for which statistics are available from the Centers for Disease Control and Prevention.

There was nothing unique about that year – gun-related suicides have outnumbered firearm homicides and accidents for 20 of the last 25 years. In 2005, homicides accounted for 40 percent of gun deaths. Accidents accounted for 3 percent. The remaining 2 percent included legal killings, such as when police do the shooting, and cases that involve undetermined intent.

Public-health researchers have concluded that in homes where guns are present, the likelihood that someone in the home will die from suicide or homicide is much greater.

Studies have also shown that homes in which a suicide occurred were three to five times more likely to have a gun present than households that did not experience a suicide, even after accounting for other risk factors.

In a 5-4 decision, the high court on Thursday struck down a handgun ban enacted in the District of Columbia in 1976.

One public-health study found that suicide and homicide rates in the district dropped after the ban was adopted.

The American Public Health Association, the American Association of Suicidology and two other groups filed a legal brief supporting the district’s ban. The brief challenged arguments that if a gun is not available, suicidal people will just kill themselves using other means.

More than 90 percent of suicide attempts using guns are successful, while the success rate for jumping from high places was 34 percent. The success rate for drug overdose was 2 percent, the brief said, citing studies.