“Slutty” behavior is good for the species. That is the conclusion of a new wave of research on the evolutionary drives behind sexuality and parenting.

Women everywhere have been selflessly engaging in trysts outside of matrimony. And they have been doing it for a good long time and for excellent reasons. Anthropologists say female promiscuity binds communities closer together and improves the gene pool.

More than 20 tribal societies accept the principle that a child could, and ideally ought to, have more than one father, according to Pennsylvania anthropologist Stephen Beckerman. “As one looks, it begins to crop up in a lot of places,” says Beckerman, who has reviewed dozens of reports on tribes from South America, New Guinea, Polynesia and India as co-editor of the newly released book, “Cultures of Multiple Fathers.”

Less than 50 years ago, Canela women, who live in Amazonian Brazil, enjoyed the delights of as many as 40 men one after another in festive rituals. When it was time to have a child, they’d select their favorite dozen or so lovers to help their husband with the all-important task. Even today, when the dalliances of married Barí ladies in Columbia and Venezuela result in a child, they proudly announce the long list of probable fathers.

In other words, the much-touted evolutionary bargain of female fidelity for food — trotted out by evolutionary psychologists with maddening regularity — just doesn’t hold up.

“This model of the death-do-us-part, missionary-position couple is just a tiny part of human history,” says anthropologist Kristen Hawkes, who has spent years studying the foraging habits of the Aché, a Paraguayan people, and the North Tanzanaian tribe Hadza, who also celebrate a rich love life. “The patterns of human sexuality are so much more variable.”

American college students still learn that human society is based on the age-old economic contract between the sexes: Men hunt and women raise children. Fathers provide meat for the family, and in exchange, moms offer fidelity and the guarantee of paternity. While men — who produce millions of sperm — are inveterate philanderers, gals, stuck with relatively few eggs that require a significant investment, tend to be choosy and coy. Men therefore are biologically prone to spreading their seed far and wide, while women focus on finding the perfect pop.

“This evidence is a real thumb in the eye for that view,” says Beckerman.

Anthropologists claim, good judgment aside, evolution has nudged women a bit toward promiscuity and sexual adventure. In all well-studied primates, females exhibit a polyandrous tendency when given the opportunity to stray. Some who cheat appear to be more fertile, and the offspring of most are more likely to survive. Fooling around appears to have helped our ancestral mothers equip their little ones for success — the sexual equivalent of reading to them every night or enrolling them in the after-school chess club.

“Women tend to do things that are associated with the welfare of their kids,” Hawkes says.

In contrast to the sex-for-food model, multiple and various sexual pairings have little to do with adding to the larder in the groups Hawkes studies. The average Hadza hunter, who can only bring in a big game carcass once a month, has to share his kill with everyone. His wife and kids just have to get in line. Extra mates add a little genetic diversity. But Hawkes says females likely hook up with multiple males for safety more than any other benefit — a mother’s strong emotional bonds with more than one fellow provide an extra protective hand in times of danger.

An economic incentive promotes female infidelity in Barí society. All of the Barí children who had more than one father were more likely to survive into adulthood, fortified by small gifts of fish and game in times of scarcity. Multiple dads also help ensure a child’s health. Since a father is necessary to blow tobacco smoke over the little one’s body if he or she falls ill, the more potential volunteers the better.

Elderly Barí ladies chuckle and nudge each other as they talk about a lifetime of lovers. But the pleasure wasn’t only their own. The men benefited, too. It turns out Barí males can’t count on a very long life. The Venezuelan tribe suffers from bouts of malaria and tuberculosis and, until 1960, was repeatedly attacked by landowners, oil companies, and homesteaders in the region. Most of the victims have been reproductive-age males. “You know that if you die, there’s some other man who has a residual obligation to care for at least one of your children,” Beckerman explains. “So looking the other way or even giving your blessing when your wife takes a lover is the only insurance you can buy.”

Even evolutionary psychologists, stout defenders of the meat-for-fidelity model, are beginning to acknowledge the benefits of women’s “slutty” behavior. University of Texas psychologist David Buss gives the most credit to what he terms “mate insurance,” a backup replacement in case the male partner doesn’t survive.

Social approval of infidelity does not, however, imply a corresponding devaluation of marriage. “They’re very, very faithful,” says Beckerman’s co-author Paul Valentine about the Curripaco, who live on the border between Columbia and Venezuela. The tribe believes that conception is a process that requires a lot of work, and the men are quick to take credit for their joint labors. “They say, ‘Hey, this is really hard work having a baby,'” Valentine says. “And they really put on a smug look.”

Physiological data supports the theory that women have been sleeping around for centuries. For starters, men have evolved to compete in their partner’s reproductive tract. Human males have large testicles that manufacture plenty of semen, especially when they reunite with their wives after separation. Their sperm includes coil-tailed versions that block instead of carry the ball. Females cooperate when they want to — more often with their lovers than with their mates, according to one study. Women retain slightly more sperm after orgasm, and in the throes of excitement may even draw the virgin swimmers up through the cervix and into the uterus, according to British sexologist R. Robin Baker.

Still, David Buss places most of the blame for all this wanderlust on the guys. Bottom line, sperm are cheap and eggs are expensive, he says. He cites his own 1993 studies of college undergraduates. Women said they’d like maybe up to five partners in a lifetime. Men in various surveys ranged from 18 up to 1,000. Sure, both sexes have one-night stands. Both also can mate for life. But men tend toward variety and women will most often stay true to the stable, dependable provider, Buss claims. “Women typically have high standards in either case; men are willing to go down to the tenth percentile (for short-term partners), as long as she can mumble,” he says.

Anthropologists are not so sure. Some say today’s emphasis on female monogamy may have more to do with socio-economic trends than evolutionary instincts.

Extramarital trysts were a way of life for the Canela — until the encroachment of outsiders. “Multiple lovers, that’s just part of the life. It’s recreation, just like races and running. It’s all done in the spirit of joy and fun,” says William Crocker of the Smithsonian Institution, who has studied the Brazilian tribe since 1957. When a woman got pregnant with her husband, she would go out to find as many as five more “fathers” for her fetus. Since every bit of semen was believed to contribute to the baby, a dedicated mom looked for a variety of desirable traits in her lovers: sexual skills, good looks, oratory talents, top-notch singing abilities — and naturally, a good provider.

Crocker says the Canela’s sexual customs began to disappear after the arrival of traders, who brought in material goods such as machetes, axes, pots and pans, introducing the idea of exclusive ownership. The missionaries came next. The evangelists, who arrived in the early 1970s, translated the Bible into Canelan and did their part to discourage the tribe’s sexual intimacy.

The pattern is repeating itself with the Barí as missionaries import rural Catholic values. Beckerman says, “I suppose it doesn’t mean there’s any less fooling around, it’s just that the fathers don’t take responsibility for it and the mothers don’t admit it.”

Modern relationships are not all that different. High infidelity, remarriage and divorce rates may have less to do with modernity than with our collective sexual past. “It makes the variation we’re seeing in modern society so much more understandable,” Hawkes says.

If the anthropologists are right, monogamy may well be counter-evolutionary or an adaptation to modern life. Or perhaps the nuclear family has always been more of an ideal than a reality.