The traits are the self-obsession of narcissism; the impulsive, thrill-seeking and callous behaviour of psychopaths; and the deceitful and exploitative nature of Machiavellianism. At their extreme, these traits would be highly detrimental for life in traditional human societies. People with these personalities risk being shunned by others and shut out of relationships, leaving them without a mate, hungry and vulnerable to predators.

But being just slightly evil could have an upside: a prolific sex life, says Peter Jonason at New Mexico State University in Las Cruces. "We have some evidence that the three traits are really the same thing and may represent a successful evolutionary strategy."

Jonason and his colleagues subjected 200 college students to personality tests designed to rank them for each of the dark triad traits. They also asked about their attitudes to sexual relationships and about their sex lives, including how many partners they'd had and whether they were seeking brief affairs.

The study found that those who scored higher on the dark triad personality traits tended to have more partners and more desire for short-term relationships, Jonason reported at the Human Behavior and Evolution Society meeting in Kyoto, Japan, earlier this month. But the correlation only held in males.

James Bond epitomises this set of traits, Jonason says. "He's clearly disagreeable, very extroverted and likes trying new things - killing people, new women." Just as Bond seduces woman after woman, people with dark triad traits may be more successful with a quantity-style or shotgun approach to reproduction, even if they don't stick around for parenting. "The strategy seems to have worked. We still have these traits," Jonason says.

This observation seems to hold across cultures. David Schmitt of Bradley University in Peoria, Illinois, presented preliminary results at the same meeting from a survey of more than 35,000 people in 57 countries. He found a similar link between the dark triad and reproductive success in men. "It is universal across cultures for high dark triad scorers to be more active in short-term mating," Schmitt says. "They are more likely to try and poach other people's partners for a brief affair."

Barbara Oakley of Oakland University in Rochester, Michigan, says that the studies "verify something a lot of people have conjectured about".

Christopher von Rueden of the University of California at Santa Barbara says that the studies are important because they confirm that personality variation has direct fitness consequences.

"They still have to explain why it hasn't spread to everyone," says Matthew Keller of the University of Colorado in Boulder. "There must be some cost of the traits." One possibility, both Keller and Jonason suggest, is that the strategy is most successful when dark triad personalities are rare. Otherwise, others would become more wary and guarded.