After 2001, the U.S. crime rate started tracking downward, and then death penalty opinions for specific groups such as women, nonwhites, Democrats and independents begin trending down, Anderson said. But for groups of whites, Republicans and evangelical Protestants, it remained high.

For those groups for which support remained high, it could have been a kind of Sept. 11, 2001, effect, Anderson said. She would like to see further study on whether people's sources of news — such as those that hype crime — affect their opinion on the death penalty.

"You have groups that maybe don't understand the crime rate has actually gone down because from their news coverage it doesn't seem like it," she said.

Anderson said there's also a hypothesis that as you get older you become more conservative. But the study suggests that peaks toward the end of middle age, then trends back down.

She and the others speculated that support of the death penalty through middle age could result from having a family to protect, but as people age and begin facing their mortality, they may think of death and capital punishment differently.

Other influences they found on how people determine support or opposition to the death penalty were religion, race and political ideology.