Romeo and Juliet, Tarzan and Jane, Ozzie and Harriet, and of course, Adam and Eve— adorable, admirable, appealing, … and one thing more: mythic. In one form or another, the iconic image of the happy, naturally monogamous, heterosexual twosome has been with us for a long time, but it has always been a fiction. There is something downright inspiring about that First Couple sharing biblical bliss— albeit, nonsexual— in the Garden of Eden; and then, when banished, striding out hand in hand, ready to confront the world as a committed couple. But evolutionary biologists and anthropologists know that reality is otherwise. Not only are we the products of evolution rather than special creation (corrective reality 1), but human beings evolved in a regime in which mating was often polygamous; and, moreover, we carry stigmata of this circumstance with us today (corrective reality 2, and the subject of this book).

Just as the Biblical account of Adam and Eve has been superseded by the evolutionary account of how we actually came to be, the Edenic myth of mutually embraced monogamy is, right now, being replaced by yet more biological understanding of how our sexual selves actually evolved and how as a result men and women are inclined— when possible— to mate with more than one member of the opposite sex. We never actually lived in any Garden of Eden, but many of our ancestors— not that long ago—were polygamous. This fact, and the often-hidden consequences of our inherited polygamous inclinations, have had a number of unexpected and largely troublesome effects.

Only recently, however, have anthropologists, evolutionary biologists, and psychologists understood what has been going on and why, not just in our sex lives but in other seemingly disconnected ways that human beings go about being, well, human. The biological reality is that we weren’t “made for monogamy,” despite the preferences of straight-laced (and often hypocritical) preachers; and not for free-spirited sexual adventurism either, despite the fervent desire of those seeking to justify a chosen “swinging” lifestyle.

. . .

In an earlier book, The Myth of Monogamy: Fidelity and Infidelity in Animals and People, my wife Judith Eve Lipton and I coauthored the first accessible written account of findings based on DNA fingerprinting, which showed that sexual monogamy (as opposed to “social monogamy”) is exceedingly rare—in humans as well as other animals. In the book you are now reading, I build on that, incorporating research that has accumulated since 2001, clearly demonstrating the underlying prevalence of polygamy as the default setting for human intimacy. I also develop some of the wide-ranging consequences of this new understanding of human nature, presenting material that is for the most part widely acknowledged by biologists and anthropologists but is made available here, to the general public, for the first time.

. . .

Polygamy isn’t only troublesome ethically, socially, economically, and legally: it’s also complicated in its own right. It comes in two forms: polygyny, in which one man maintains a “harem” of wives with whom he mates and typically produces children; and also its mirror image, polyandry, whereby one woman mates with more than one man. Polyandry is exceedingly rare, at least as a formally constituted mating system complete with recognized marriages in which one wife has multiple acknowledged husbands. (It is best known from certain tribal groups in Tibet, the Marquesa Islands, and some regions in the Brazilian rainforest and pretty much nowhere else.) Polygyny, on the other hand, is much more widely distributed, both geographically and throughout recorded history; more important, its imprint is detectable in some of our deepest inclinations.

The reason “polygamy” is often used as a synonym for “polygyny” is probably because polyandry is so rare that polygamy and polygyny are almost the same thing. But they aren’t. In fact, although polygyny is well known— not only because it is very common but because it represents a pattern that is often culturally encouraged— the reality is that polyandry is also common: not as a formally constituted marital system but rather simply because women, like men, are also prone to having multiple sexual partners. It’s just that in the context of patriarchal social traditions (which is to say, most systems!), polyandry is typically hidden, whereas polygyny is more overt.

Both mating variants, polygyny and polyandry, are fascinating as well as fundamental to being human. Taken as a whole, polygamy— that is, polygyny plus polyandry— is politically fraught, emotionally disruptive, economically confusing, illegal (at least in modern Western societies, including the United States): and yet it is something to which all of us, like it or not, are biologically predisposed.

Polygyny in particular is surrounded by myths and misunderstandings, even though its roots go deep into our evolutionary history; and its impact can be seen not only in our anatomy and physiology but also in many of our behavioral predispositions (including sexual preferences as well as inclinations for violence and many other things) plus certain seemingly unbiological traits, such as parenting and creativity, and maybe even homosexuality and monotheism.

The polygamous nature of Homo sapiens, although clearly demarcated by biology, is likely to be controversial and “politically incorrect.” But that doesn’t make it any less valid, or render it any less important as something to acknowledge and understand.

. . .

Genomic studies based on retrieved fossil human DNA has further confirmed our species’ primordial polygyny.27 Looking at the mid-range human fossil record, there is a disparity between the relatively low levels of Y- chromosome diversity (the Y chromosome is inherited from fathers only) and the greater diversity in mitochondrial DNA (which is inherited only from mothers). The researchers concluded that this is most parsimoniously explained by the likelihood that “until recently only a few men … contributed a large fraction of the Y- chromosome pool at every generation.” In other words, our ancestors were mostly polygynous, with the effective breeding population of women being substantially greater than that of men— because whereas most women reproduced, a small number of men were disproportionately doing so. Then, over time, “the variance of their [the males’] reproductive success … decreased, through a recent shift from polygyny to monogamy,” as more men began contributing their own Y chromosomes to the gene pool. The researchers conclude—based purely on the DNA evidence— that “over much of human prehistory, polygyny was the rule rather than the exception.”

Their estimates are that much of this transition from polygyny to increasing frequency of monogamy occurred between 10,000 and 5,000 years ago. Insofar as this is accurate, the fact that so many traditional societies remained polygynous even into modern times speaks strongly to our species- wide polygynous stubbornness. Our shift from polygyny to monogamy, in short, is a recent evolutionary development and is still a work in progress.

The word “monogamy” as used here— and pretty much everywhere— should be corrected to one wife at a time, since by far the most frequent pattern (for women as well as for men) has long been “serial monogamy,” which could as well be stated as “serial polygamy.” As I’ll discuss later, human beings also show a powerful inclination for pair bonding (which nonetheless doesn’t preclude a parallel penchant for polygamy). It is exceedingly rare for either a man or a woman to have literally one and only one sexual partner in his or her entire life. Pretty much all men strive to be polygynously mated, whether formally and legally, where permitted, or informally, simply as a result of their personal behavior (concubines, prostitutes, or infidelity)—resulting in de facto polygyny.