Liam Farrell, a general practitioner from Britain, uses this analogy to describe the restrictions felt by people forced into monogamous relationships. "How do you restrain the six tons of throbbing horsepower between your legs?" Farrell adds.1 People, especially women, have been asking this question for years: How do you restrain sex drive? Many societies have attempted to institutionalize monogamy, but they have often failed--either openly or subversively. Scientists have now begun to address this question from a biological perspective. The results of this research startlingly contrast traditional views of monogamy advocated by culture and religion (2). Several classic features characterize monogamy. In a monogamous relationship, a breeding pair shares the same nest and territory where they are frequently in contact. Monogamous males participate in parental care, and intruders of either sex are rejected (3). Biologists believe that monogamy evolved in those species whose fathers could help rear the offspring (e.g., birds). Biologists have observed birds to be the most monogamous animals in the world--much more so than humans. Over ninety percent of bird species pair up exclusively to mate and rear chicks together because both mother and father are equally able to provide food for the chicks (4). The theory that monogamy originated from an equal partnership in "child-rearing" explains why only a small percentage (approximately three-percent) of mammals are monogamous (3). In mammals, females alone produce milk for the young. The percentage of primates that are monogamous is, however, considerably higheræperhaps as large as fifteen percent. Most other mammals practice either promiscuity or polygyny (4). In promiscuity animals have many mates and, furthermore, no relationships with those mates beyond the act of sex. The reproductive strategy for promiscuous species seems to be the spreading of sperm widely and abundantly. Males who have faithful mates, whether one or a harem of faithful females, use their energy instead to secure the territory for their partners and offspring, thereby giving the young ones a better chance to grow to adulthood (4). Polygyny describes a situation in which a single male mates with multiple females. Some scientists have even placed Homo sapiens in this category by citing studies of human culture in which four in five societies practice polygyny. The difference is that species other than humans do not make the pretense of faithfully bonding for life (2, 4). Researchers observe that these different strategies are revealed in the size of an animal's testes. Chimpanzees and gorillas provide a nice example. Chimps are promiscuous, and gorillas are polygynous. Male gorilla bodies are about four times bigger that male chimp bodies, but the chimps' testes are four times larger than gorillas'--sixteen times larger, proportionally.(4) The chimps have larger testes because it is important for them to have sex with many females and spread their seed. They are not committed to any one female, and they have no assurance of the survival of their children because they are not obligated to care for any particular one. Gorillas, on the other hand, are committed to a group of females. They devote less energy to spreading their genes and more to protecting their territory and providing for their females and their children. Thus, they do not need larger testes (4). These same ratios are noted in butterflies, frogs, and many others. Based upon this criterion of relative testes' size, humans fall into the category of monogamous/polygynous (4). An alternative hypothesis has also been presented to explain the origin of monogamy. According to advocates of this theory, monogamy has evolved more frequently in the absence of paternal care than in its presence. Monogamy does not normally occur in species where female habitat ranges are large. Thus, the most common feature of mammalian monogamy is that it evolved where females were solitary and occupied small, exclusive ranges, enabling males to monopolize them (5). Regardless of the why monogamy evolved, Dr. Stephen T. Emlen of Cornell University is also careful to point out that "true monogamy actually is rare." Genetic testing techniques indicate that even the most apparently devoted of partners in nature often mate with others, visiting nearby nests or dens or clans to share the sexual company of completely strangers (2). Emlen described a great difference between "social monogamy," where mating pairs work together to raise their young, and "genetic monogamy," where parents are faithful sex partners. Social monogamy is relatively common among monogamous species, but genetic monogamy is the exception. Overall, only ten percent of the birds and mammals that seem to mate for life are actually faithful to their partners (2). Dr. Emlen noted that, among primates, there are only two monkeys--the marmoset and its South American cousin, the tamarin--that are genetically monogamous. All other primates--monkeys, apes, and humans--often mate outside their socially monogamous partnerships (2). Among birds, faithful sex partnership had been thought for many years to be widespread. Only recently, using advanced genetic fingerprinting techniques to determine parentage, have scientists determined that some thirty percent of the time, male birds help to raise chicks that are not their own. The mother bird, in other words, lays eggs of mixed lineage--sure evidence of her promiscuity (2). The same phenomenon exists in humans. One Oregon study went so far as to suggest that the male partner of the parental pair did not sire about ten percent of children in the study (2). Many theories propose possible causes of sexual infidelity. Experts say that female birds and mammals may be biologically driven to stray in order to pick up the best genes possible for her offspring. Several studies have shown that "females socially bonded with high-quality males do not have copulations outside the pair bonds." Essentially, such females believe that they already have the best of what's around, so why should they look for better? (2) Biologically speaking, females may be getting the best of both worlds by cheating: the female has a dependable mate to help raise her offspring, and she allows fresh genes to be introduced into the family (2). Males, on the other hand, may be driven by an impulse to father as many offspring as possible. They are biologically driven to stray by the desire to pass their genes through as many offspring as possible. Among species such as lions, mountain gorillas, and grizzly bears, this drive to influence the future genetically leads dominant males to kill and even eat the young of competing males. It is important to remember, however, that impulses that drive humans to seek sex outside their partnerships are far more complex (2). As a consequence of this complexity, groundbreaking research in molecular biology seeks to explain not only why people are unfaithful, but also why they are faithful at all. Dr. Thomas Insel of Emory University (see related interview) has identified two closely related neuropeptides, oxytocin and vassopressin, implicated in the central mediation of attachment behaviors (3). These neuropeptides appear to be important for the initiation of pair bonds, which is an essential component of monogamy. Oxytocin, vassopressin, and their receptors have been positively identified in voles, and the existence of oxytocin and vassopressin has been verified for humans. They have also been associated with sexual intercourse in men and with giving birth in women (3). However, the exact mechanism of their employment is not yet understood. It is important to note that sexual intercourse is neither necessary nor sufficient for human pair bonding; thus, it is certain that these two peptides are not the only chemical processes affecting monogamy. It may be that differing concentrations of these and other neuropeptides determine a tendency towards or away from fidelity (3). Do these biological explanations justify infidelity? Should one accept his/her spouse's cheating? The consensus among scientists thus far is a resounding, "No!" They caution against drawing simplistic conclusions about human biology from animal studies (2). One scientist has noted the positive effects of unsatisfied lust, and presents William Butler Yeats as a case in point. Yeats fell in unrequited love with Maud Gonne at the age of 23, and his loss inspired possibly the greatest body of love poetry in the English language: the anguish, the ruin, the grief, the lurching between melancholy and despair. We must remember, "[Humans] have a big brain; we can decide all sorts of things. Just because there's biology does not mean we are destined to follow any particular biological route (1)."