Aside from the terrifying sameness of brutality and indiscriminate destruction, there is another common denominator in the mass shootings in Aurora, Oak Creek, Columbine, Fort Hood, Virginia Tech and Newtown. All of the shooters were male.
"It suggests there's something about maleness. An aggrieved entitlement that's unique to men," said Hugo Schwyzer, a professor of history and gender studies at Pasadena City College, who has written frequently on the topic.
"Women do commit murder. There are women killing ex-lovers, killing rivals, killing small children. But they tend to kill people to whom they have a deep emotional tie," Schwyzer said.
"This killing, in which the killing doesn't matter, is male."
As Mother Jones magazine noted in a recent review of 62 mass shootings in the U.S. it reviewed since 1982, only one involved a female.
That was in 2006, when a 44-year-old former California postal worker, Jennifer Sanmarco, killed a neighbor, six employees at her old mail processing plant and then herself. She used a semiautomatic handgun.
The magazine apparently omitted an Oct. 30, 1985, shooting spree in which a mentally disturbed woman, Sylvia Seegrist, 25, killed two men and a 2-year-old, and wounded seven others during a rampage in a Springfield, Pa., mall.
The gender disparity is just as glaring when looking at homicides in general.
According to supplementary homicide reports for 1976 through 2005 from the Federal Bureau of Investigation, males committed 88.
Looking deeper, men committed 91.3 percent of gun homicides from 1976-2005; women committed 8.7 percent. Men were responsible for 93.5 percent of homicides with multiple victims; women were responsible for 6.5 percent.
More recently, the FBI's Uniform Crime Reports for 2010 show that 6,276 men were arrested for murder and non-negligent manslaughter that year, compared to 751 women.
"There's still a huge difference in the way we raise boys and girls," said New Haven psychotherapist Linda Barone.
"Girls tend to define themselves in relation to each other. Boys often define themselves in competition with each other. Being a loner - that isn't encouraged at all in women and girls.
"Women tend to turn their anger inward and blame themselves for things, traditionally. Boys are expected to have fist fights and to express anger physically," Barone said.
Yet other scholars contend there's also a biological factor in gender and violence.
Harvard University psychologist Steven Pinker, author of "The Better Angels of Our Nature," said that in most primates and societies, gender differences regarding violence emerge as early as toddlerhood.
"Though the exact ratios vary, in every society it is the males more than the females who play-fight, bully, fight for real, carry weapons, enjoy violent entertainment, fantasize about killing, kill for real, rape, start wars, and fight in wars," Pinker asserts in his book.
"Not only is the direction of the sex difference universal, but the first domino is almost certainly biological."
Reached by email, Pinker said the Sandy Hook Elementary School shootings were a tragic example of this gender difference.
"Most rampage killings such as that in Newtown may be even more male-biased because they are triggered by the kinds of motives that especially differentiate the genders - violence for dominance and revenge, which is largely a guy thing," Pinker said.
Other research suggests a more complex picture of emotional expression between genders.
Tara Chaplin, an assistant professor of psychiatry at the Yale School of Medicine, recently was the lead author of a comprehensive analysis of every study on childhood and adolescent emotional expression in the past 30 years. The analysis gathered data from 166 studies, with 21,709 participants ranging in age from birth to 18.
The analysis yielded a nuanced set of results, with several surprises.
For example, while there were significant gender differences in processing emotion overall, those differences remained small, Chaplin said. Also, differences seemed to peak by age 12.
"There are a lot of assumptions in society about gender differences in emotional expression, but there's been no comprehensive review until now," Chaplin explained.
But here's where it gets more complicated. Chaplin found that boys' and girls' expression of anger diverges depending on their surroundings. At home, with loved ones, both genders feel freer to show anger. But around their peers, boys show their anger more than girls do.
Likewise, when they're alone watching a movie or video, boys externalize their anger more than girls.
"We were really surprised by this, and we have no great explanation for it," Chaplin said. "There should be no social pressure to show anger when you're alone."
Also, Chaplin was quick to point out that expressing anger is not necessarily a bad thing. In moderation, it builds persistence, assertiveness and independence.
"However, there may also be risks attendant with this tendency to express externalizing emotions in young boys," Chaplin wrote in her paper.
"If anger is expressed at a high level and is expressed to the exclusion of emotions like fear or empathy, and certain stressors are present, there could be a risk for the development of conduct problems, such as aggressive behavior and disregard for rules of conduct."
That's where parents, educators, therapists and an enlightened public can help, experts said.
"A big part of this is about the way in which we deal with young men and anger. And rage. And pain," said Schwyzer.
"Women are permitted public displays of emotion that men are not. We need to find ways for men to have healthy expressions of anger."
Contact reporter Jim Shelton at 203-789-5664.