Moody Bitches: The Truth about the Drugs You’re Taking, the Sleep You’re Missing, the Sex You’re Not Having and What’s Really Making You Crazy.... Julie Holland

be more likely to cheat on their husbands when they’re ovulating, thanks to the surge in testosterone, the hormone of novelty.

      Monogamy in Nature? Not So Much

      Very few animals are actually sexually monogamous: 3 percent of mammals, and only one in ten thousand invertebrates. Pair-bonding is rare among mammals; only 3 percent rear their young in this way. “But what about penguins and swans?” you ask. Well, sorry to burst your bubble, but they don’t have sex with one partner forever. Penguins are monogamous only until their eggs hatch; the next year they choose new mates. Over his lifetime the “monogamous” penguin may have created two dozen families. And though they do pair-bond to raise their young, swan nests were found to have young from multiple fathers. In fact, when you look at the offspring of the few “monogamous” birds and mammals, infidelity is present in 100 percent of species examined. This is why I think of monogamy as unnatural. Not undesirable, not unattainable, but certainly not natural.

      So let’s look at our own family tree, where the primates are. First of all, we didn’t descend from apes; we are apes. We are part of the same family, the great apes (Hominoidea) containing gorillas, orangutans, chimpanzees, bonobos, and humans. Chimps and bonobos (formerly called pygmy chimps) are our closest primate relatives. Female bonobos and chimps mate multiple times with multiple males in a row, raising children fathered by different males. Humans and bonobos, but not chimps, have missionary sex, face to face. We both kiss deeply and look into each other’s eyes when mating. We also both carry the genetic codes for oxytocin release, which helps to bond lovers. Chimps stick to rear entry; the female’s vulva faces back, not forward, as in bonobos and humans.

      In bonobo troops, the female status matters more than the male status, with older females outranking the youngsters. Bonobos are significantly less aggressive than chimps. Sex is used to keep social order, and genital rubbing between female bonobos is common, used to solidify female bonding. It should be noted here that the bonobo clitoris is three times bigger than ours, taking up two-thirds of the vulva, and positioned optimally for ventral stimulation, so all that rubbing has a big payoff. Suffice to say, none of this matriarchy or genital rubbing to keep the peace is seen in chimps. Also, only humans and bonobos have a significant percentage of homosexual sex and, most important, have sex outside of ovulation, a rarity in the animal kingdom. Pair-bonded monogamous animals have infrequent sex and only for reproduction. Sex to keep the peace or solidify the relationship is not practiced.

      Among all the social, group-living primates, monogamy is not the norm. The one ape that is monogamous lives in treetops and is solitary, part of the lesser ape family, the gibbon. As humans are the most social of all the primates, except for perhaps the bonobos, it is unwise to assume we’d naturally be monogamous. Body dimorphism (different sizes for different genders) is correlated with male competition for mates. If men and women were monogamous, we’d be the same size, as the gibbons are. If we were completely polygynous (men taking multiple female partners), men would be twice the size of women, as male gorillas and orangutans are. Chimps, bonobos, and human males are all around 10 to 20 percent larger and heavier than females, which implies we have similar rates of promiscuity.

      The Biology of Fidelity—Vasopressin and Prairie Voles

      The well-studied monogamous prairie voles maintain a single pair bond while raising several litters. As in humans, sex triggers oxytocin release in the females and vasopressin release in the males. The meadow voles, on the other hand, are solitary, asocial, and promiscuous. The vasopressin receptor is quite different in this species. When genes from the monogamous male prairie voles were injected into the brains of the promiscuous male meadow voles, more vasopressin receptors were formed, and the animals started to fixate on and mate with one female vole only.

      Vasopressin in the males peaks during sexual arousal. It not only triggers partner preference but is also involved in male parental care. I know what you’re thinking: Can I inject vasopressin into my husband? No, and some guys have more vasopressin than others. There are different genes that code for vasopressin, and some men have certain genes that others don’t have. Men who have a gene variant called 334 score lower on feelings of attachment for their spouses and are more likely to have experienced a marital crisis during the past year or to be in a relationship without being married.

      Testosterone levels not only affect sex drive and sexual response but also have a lot to do with fidelity and parenting impulses. Married men and fathers have lower testosterone levels than single and childless men. Right after his child is born, a man’s testosterone levels might fall as much as 30 percent. Men who maintain multiple female partners (polygyny) have higher testosterone levels than monogamously married men. Not surprisingly, married men with higher testosterone levels have sex more frequently than those with lower levels, and men who cheat have higher testosterone levels than those who don’t. Unconsciously, women may know this. In one study, women rated men with lower voices (more testosterone) as being more likely to be unfaithful and were more likely to select more masculine men with lower voices as short-term, rather than long-term, partners.

      The Coolidge Effect

      Named for Silent Cal, the story goes that he and the missus were separately being shown around a farm. Upon hearing that the rooster mated dozens of times a day, Mrs. Coolidge said to the guide, “Tell that to my husband.” Later, when he heard his wife’s remark, the president asked, “With the same hen?” “Nope. It’s a different hen every time.” “Tell that to my wife.”

      The Coolidge effect, that varying the sex partner invigorates the libido, has been documented in many male mammals, including humans. But it turns out that female primates are aroused by novelty as well. Unfamiliar males are more attractive than the known quantity. “The search for the unfamiliar is documented as a female preference more often than is any other characteristic.” Nature has bred philandering into our genes, enhancing the mating strategy of more copulations in order to increase the likelihood of passing those genes on. If guys who are players make more babies, there will be more players in the gene pool.

      Ever wonder why men who seem to have it all—fame, fortune, and a loving family—throw it all away for a bit of strange? It’s nature, trumping both reason and willpower. Novelty is the strongest attractor. Many of us are “novelty seeking.” We enjoy new restaurants, new music, new friends, and new hobbies. Research suggests that people who cheat are not only novelty seekers but also more likely to be extroverts than their partners are. They’re also simply more easily bored.

      The Space Between

      Couples who spend all their time together may end up being too close. Like a fire deprived of oxygen, sexual energy sputters when there’s no room to breathe. You each need to bring something separate and “other” to the partnership, which means you have to go out there and have your own experiences. Girls’ night out is good for both of you. Don’t confuse love with merging. Eroticism requires separateness;