Politics, it is said, makes strange bedfellows. The phrase actually comes, however, from Shakespeare’s play The Tempest, and was originally somewhat different. A drunken jester named Trinculo has just been shipwrecked on Prospero’s magic island. The storm is still raging, and Trinculo seeks shelter under the ragged garments of the resident monster, Caliban. “Misery,” says the resigned but desperate Trinculo, “acquaints a man with strange bedfellows. I will here shroud till the dregs of the storm be past.” Centuries later, in 1850, one Charles Dudley Warner, editor of the Hartford Courant, modified Trinculo’s tribulation to “politics makes strange bedfellows.” Equating politics with misery, it appears, is nothing new. Our point is that biology, too, creates strange bedfellows, especially when it comes to mating and marriage.

It is endlessly fascinating to learn who ends up in bed with whom, and for how long, and with what secrets. There are always great surprises, scandals, and scenes. The princess is betraying the prince with a dashing horseman, as the prince betrays the princess as well. The heir apparent falls for a “common” woman and gives up his kingdom. The president has a messy sexual relationship with a young intern. One gorgeous movie star abandons his lovely wife and children for another spectacular movie star—of one sex or another. Other celebrities boast of their promiscuity and sexual track records. This is the stuff of People magazine and other tabloids, because—let’s face it—people want to know who are the strange bedfellows du jour.

It seems that, by contrast, monogamy is boring, not only for each couple but for the gossiping public. Who cares if two people have been faithfully devoted for decades? It is the departures that make the headlines, except for those rare cases in which a mateship persists for 75 years or so. The point of this book, however, is that monogamy is interesting in itself, and that the strange bedfellows who make it permanent may not necessarily do so in a context of misery. In the 1960s, a pair of our friends were accused of living in “unmarried bliss.” The question presents itself: Is it possible to be married and blissful, or at least monogamous and satisfied? Is it possible for human beings to be monogamous at all?

Certain situations—typically, demanding ones such as storms or politics—predispose toward outcomes that involve unlikely combinations. The psychology of mating also creates situations of this sort, because if nothing else, it is highly unlikely that the inclinations of any one individual will be identical to those of another, whether or not the two are matrimonially united. Instincts and desire are actually composites of many things, some of them pointing in contradictory directions. Specifically, as we shall see, human biology predisposes Homo sapiens of both sexes to seek sexual variety and multiple partners, thus undermining monogamy. But at the same time, the chief argument of this book is that biology does not preclude monogamy; in fact, there are certain evolutionary factors that resonate strongly with monogamy, offering shelter from infidelity’s storms.

And make no bones about it: infidelity makes waves. This is not to say that open relationships or other sorts of arrangements don’t also cause mayhem, but rather that infidelity (cheating, deception, philandering) nearly always causes substantial trouble for human beings, as for other animals. Deliberate and careful monogamy can provide shelter from these storms of betrayal, but not without taking its own toll.

Monogamy itself can be tempestuous, among animals no less than human beings, and just as biology often threatens to capsize so many matrimonial boats, it also—ironically, but no less genuinely— offers at least the prospect of stability. Either way, everyone is Trinculo, seeking refuge under less than perfect circumstances. And to understand Trinculo, it helps to look hard at Caliban, and perhaps to conclude, as did Prospero in The Tempest: “This thing of darkness I acknowledge mine.”