Convicted murderer Jodi Arias Jodi Arias/MySpace It took only one juror to spare Jodi Arias the death penalty for the brutal murder of her ex-boyfriend Travis Alexander in 2008. She will, however, spend the rest of her life behind bars, without the possibility of parole.

Considering the United States has executed only 13 women in the last 40 years, a death sentence would have been highly unusual.

Women committed less than 10% of all murders in America between 2000 and 2010, a Wall Street Journal analysis of crime data found. Women defendants, however, only make up 2% of death row, according to a recent report by the NAACP.

Even fewer women actually get executed, Death Penalty Information Center executive director Richard Dieter told Business Insider.

"There's just less enforcement of the death penalty at almost every stage for females," he said.

Two major factors contribute to the low number of women who get capital punishment: the nature of the crime and how juries view women in general. The death penalty is often used for killers who also commit other felonies like robbery or rape, law professor Victor Streib has previously told the LA Times.

Many of the murders women commit, on the other hand, involve people they're related to.

While women commit about 10% of murders, they were responsible for 35% of murders of intimate partners between 1980 and 2008. Most juries consider these crimes of passion arising from disputes — one-time offenses, Dieter said. Because of the high rate of domestic violence against women, though, juries don't give men the same benefit of the doubt.

On the other hand, most states consider killing a child an aggravating factor, or a reason for prosecutors to seek the death penalty. Hiring someone to do the work could also land a woman on death row. "If a woman hires someone, there's a coldness, a calculation. It's different than something that arises out of an argument," Dieter said.

Teresa Lewis, for example, plotted to kill her husband and stepson for the insurance money. "Instead of pulling a trigger on a gun, she pulled a couple of young men in to pull the trigger for her," prosecutor David Grimes told a judge at the time, The Washington Post reported. She was the first woman Virginia sentenced to die in more than 100 years.

But the second factor — the jury's perception of the "fragile" female psyche — can overpower aggravating factors. "It's just easier to convince a jury that women suffer emotional distress or other emotional problems more than men," Streib told the LA Times.

Take Susan Smith. She killed her two sons by backing her car into a lake while they sat in the backseat. But when the jury heard about her abusive childhood, they took pity on her, Dieter said. She only got a life sentence — with parole. In TruTV's coverage of her story, the headline reads: Child Murderer Or Victim?

"These 12 people [the jury] are asked to see if this person has any redeeming qualities. And they often see their own mother or wife or grandmother, not someone who will continue to be a threat to society," Dieter said. "Jurors just see women differently than men."

Of course, most women aren't going to argue for gender parity in the death penalty, Dahlia Lithwick has written in Slate. Only 59% of women favor the death penalty compared to 67% of men, according to a 2013 Gallup poll.

"For equality's sake, you think that women would want the death penalty pursued more often," Dieter said. "But of course, they don't."