A year ago I proposed a book to several publishers about terrorism and polygamy. They thought I was crazy. Polygamy? What did that have to do with anything?

A year later, it’s an HBO sitcom about polygamy — Big Love. It’s hard to keep up these days.

With its penchant for picking the locks off civilization, the entertainment industry is presenting us with polygamists as “just plain folks.” It’s not clear where all of this is going to lead. Will polygamy become the new homosexuality? Will Massachusetts legalize it? Will college students — those sacrificial lambs on the cutting edge of each new trend — start practicing it? Is polygamy the new chic?

New York Times columnist John Tierney has already taken a libertarian stance — if people want to do it, why not let them? Now Tierney and I are good friends and I agree with almost everything he says, but here I part company. If America is going to add polygamy to its list of “Why-Nots?” we’re not going to have a civilization around here much longer.

In the March issue of The American Spectator, I have an article entitled “The Alpha Couple and the Primal Horde.” It’s what one of Freud’s critics called a “Just-So Story” about how human beings became monogamous. (Tom Wolfe liked it and has written an “Afterword.”)

And we are a monogamous species, at least in our beginnings. Hunter-gathering tribes, the original human economy, are all monogamous. Polygamy comes later, with more affluent economies.

Briefly, my conjecture is that we adopted monogamy in response to adversity. Five million years ago, a very small, polygamous ape, barely three feet tall, moved out onto the East African savannah in groups of 15-25 in search of animal carcasses — or maybe just for adventure. The whole story is too long to tell here (it’s in the issue with Mitt Romney on the cover), but basically we became monogamous for greater security. Predators were everywhere. The only safety lay in group solidarity. Monogamy became preferable because it knit the group more tightly together. In a word, it was more democratic.

Shortly after I submitted the story, I attended a lecture at Columbia by Larry Young, a neuroscientist at Emory University, who is investigating monogamy in voles. It turns out there are two species in America: the meadow vole, which inhabits woodlands east of the Mississippi, and the prairie vole, which lives out on the open grasslands west of the Mississippi. The meadow vole is polygamous. The prairie vole is monogamous. Young has even found the genes that marked the transformation. He believes the change occurred because, in open country, the prairie vole was more vulnerable to predators. As a cautious scientist he’s not jumping to any conclusions, but the analogies with human evolution sure are interesting.

Monogamy creates a society that has an inherent equality. Every male has the promise of getting a female and every female has the promise of getting a male. It gives everyone a stake in society.

But it’s not biology. Biology says that males can impregnate any number of females and that females desire the most fit and attractive males. Monogamy limits certain groups. High-status males have to be satisfied with only one mate, while low-status females have to be satisfied with lower-status males. (Last Sunday’s New York Times Magazine cover story on middle-class women mating with “sperm donors” instead of husbands is a perfect example of the discontents of monogamy. These women are dissatisfied with the choice of men available to them so they mate with a higher-status man by Federal Express.)

Both high-status men and low-status women are liberated by polygamy. As the old saying has it, men “date down and marry up.” With polygamy you can do both. Meanwhile, the losers are: 1) high-status women, who must share their mate with lower-status females, and 2) low-status men, who don’t get to mate at all.

It’s that last one that causes trouble. Every society and species that practices polygamy is plagued with a “bachelor herd” of unmated males who are very unhappy with their lot. Competition among males becomes much more violent because the stakes are so high. You either score with a couple of females or you don’t mate at all. Male fruit flies artificially bred to be monogamous have proved to be much less aggressive with other males. Take away that monogamous contract and your peaceful society disappears with it.

When 18th- and 19th-century Europeans realized polygamy was common in the “backward” portions of the world, they had an easy explanation. Polygamy was a more primitive form of marriage. Advanced societies had evolved out of it. Then they discovered the hunter-gatherers and a different explanation offered itself. Polygamous societies had remained backward precisely because they were polygamous. Polygamy creates a huge inequality where all the wealth — however little there may be of it — and all the women are concentrated among the more successful men. Exclude enough men and you have the makings of a jihad society. When there aren’t enough women to go around, it’s easy to convince low-status men there are 70 virgins waiting for them in heaven.

Monogamy is not a natural configuration. It’s a human construct. I also happen to think it’s the greatest social achievement in the history of mankind. Advanced societies never would have evolved without it.

So now the entertainment industry is going to ask, “I wonder what America would look like without it?” It’s not going to be anything any of us would recognize.