THE NOTION that men have shorter fuses than women has acquired the status of a psychological shibboleth. More than 30 years ago Stanford University psychologists Eleanor Maccoby and Carol Jacklin concluded in an influential book that sex differences were minimal in most psychological traits but considerable when it comes to aggression. This opinion has endured ever since.

Were Maccoby and Jacklin right? Recent research bears out the broad brushstrokes of their claim but reveals that women can be equally, if less dangerously, belligerent.

Mad Men
In 1995 the late psychologist David Lykken of the University of Minnesota wrote that if we could magically place all boys and men between the ages of 12 and 28 in a cryogenic freeze, we would slash the rate of violent crime by two thirds. The data bear out Lykkens thought experiment. In the U.S., the rate of violent crime for girls and women aged 10 and older is one in 56; the corresponding figure among their male counterparts is one in nine. Men commit close to 90 percent of the murders in the U.S. and more murders than women in all the countries researchers have examined, according to a 1999 report by psychologist Anne Campbell of Durham University in England.

Indeed, investigators have consistently found that short of criminal activity, men exhibit more frequent and more extreme levels of physical aggression with one exception: in domestic disputes, as we will see, the tables are often turned. In a 2004 mathematical synthesis of 196 studies (known as a meta-analysis), psychologist John Archer of the University of Central Lancashire in England found that men are more physically aggressive (by various measures) than women across all ages, with the difference peaking between the ages of 20 and 30. This sex difference extended to all 10 countries Archer examined, which included the U.S., Finland, Spain, India, Japan and New Zealand. Interestingly, researchers have found men to be more physically aggressive in their mental lives as well. Compared with women, men harbor more frequent and enduring homicidal fantasies, more often think about enacting revenge against their enemies, and report more physically aggressive dreams.

Evening the Score
Still, studies show that women are at least as prone to feeling anger as men and that they fight plenty. Instead of expressing their angry emotions with their fists, women tend to use what in 1995 psychologist Nicki Crick, then at the University of Illinois, termed relational aggression, a less overt form characterized by social manipulation, especially of same-sex peers. Popularized by such books as Odd Girl Out: The Hidden Culture of Aggression in Girls, by Rachel Simmons (Harcourt, 2002), relational aggression includes spreading rumors, gossiping, glaring, eye rolling, giving others the silent treatment, sending nasty notes or text messages behind rivals backs, excluding others from social gatherings, poking fun at the appearance of competitors, and assorted other stealth attacks. The so-called gentler sex may opt for such tactics because they are socialized to not show hostility openly and also because their relative lack of physical strength makes violence seem a less promising strategy.

Girls do not have an exclusive claim to relational aggression, however. A 2008 meta-analysis by psychologist Noel Card of the University of Arizona and his colleagues suggests that it is equally common in girls and boys across both childhood and adolescence. Other research suggests this absence of sex differences persists into adulthood.

More surprisingly, women are also just as likely as men to express hostilityin this case physicallyin the context of a romantic relationship. The popular stereotype of a domestic abuser is a man who habitually hurts his female partner. Yet research by Archer and sociologist Murray Straus of the University of New Hampshire calls this scenario into question. Surprisingly, their analyses demonstrate that men and women exhibit roughly equal rates of violence within relationships; some studies hint that womens rates of physical aggression are slightly higher. This apparent equality is not solely a result of women fighting back, because it holds even for altercations that women start. Still, domestic abuse within intimate relationships poses a greater threat to women than to men. Women suffer close to two thirds of the injuries, largely because men are stronger on average than women. In addition, women and men differ in the severity of their actions; women are more likely to scratch or slap their partners, and men more commonly punch or choke their partners.

Biology to Blame?
Until recently, most psychologists thought differences in the degree to which men and women exhibit physical aggression stemmed largely from societal reinforcement of traditional gender roles. Social factors undoubtedly account for a part of the differences. But in a study published in 2007 psychologist Raymond Baillargeon of the University of Montreal and his colleagues reveal that as early as the age of 17 months, 5 percent of boys but only 1 percent of girls engage in frequent physical aggression, such as kicking and biting. What is more, this gap does not widen between 17 and 29 months, as might be expected if environmental influences such as socialization by parents were to blame. These findings suggest that biological factorssuch as the effects of testosterone on brain functioncontribute to sex differences in violent behavior.

Bolstering this hypothesis is the fact that males are the more belligerent sex in virtually all mammalian species that biologists have studied. Even the one marked exception to this trendthe spotted (laughing) hyenamay prove the rule. The female hyena, which is more physically aggressive than her male counterpart, has higher testosterone levels than the male does.