Monogamy isn’t easy, naturally.

Right-wing pro-marriage advo­cates are cor­rect: Monogamy is def­i­nitely under siege. But not from unclos­eted polyamor­ists, ado­les­cent “hook-up” advo­cates, rad­i­cal fem­i­nists, God­less com­mu­nists or some vast homo­sex­ual con­spir­acy. The cul­prit is our own biology.

Researchers in ani­mal behav­ior have long known that monogamy is uncom­mon in the nat­ural world, but only with the advent of DNA “fin­ger­print­ing” have we come to appre­ci­ate how truly rare it is. Genetic test­ing has recently shown that even among many bird species — long touted as the epit­ome of monog­a­mous fidelity — it is not uncom­mon for 6% to 60% of the young to be fathered by some­one other than the mother’s social part­ner. As a result, we now know sci­en­tif­i­cally what most peo­ple have long known pri­vately: that social monogamy does not nec­es­sar­ily imply sex­ual monogamy.

In the movie “Heart­burn,” the lead char­ac­ter com­plains about her husband’s phi­lan­der­ing and gets this response: “You want monogamy? Marry a swan!” But now, sci­en­tists have found that even swans aren’t monog­a­mous. (Nor are those widely admired emperor pen­guins, whose sup­posed march to monogamy was mis­con­strued from another pop­u­lar movie; their domes­tic­ity lasts only for the cur­rent breed­ing sea­son — next year, they’ll find new mates.)

For some, find­ings of this sort may mit­i­gate a bit of the out­rage vis­ited on the cur­rent and future crop of adul­ter­ers du jour, recently includ­ing but assuredly not lim­ited to Eliot Spitzer, Mark San­ford, John Ensign and John Edwards. For oth­ers, it sim­ply shows that men are clue­less, irre­spon­si­ble oafs. The sci­en­tific realty, how­ever, is more nuanced, and more inter­est­ing, espe­cially for those look­ing to their own mat­ri­mo­nial future.

First, there can be no seri­ous debate about whether monogamy is nat­ural for human beings. It isn’t. A Mar­t­ian zool­o­gist vis­it­ing planet Earth would have no doubt: Homo sapi­ens car­ries all the evo­lu­tion­ary stig­mata of a mildly polyg­a­mous mam­mal in which both sexes have a pen­chant for occa­sional “extra-pair copulations.”

But nat­ural isn’t nec­es­sar­ily good. Think about earth­quakes, tsunamis, gan­grene or pneu­mo­nia. Nor is unnat­ural bad, or beyond human poten­tial. Con­sider writ­ing a poem, learn­ing a sec­ond lan­guage or mas­ter­ing a musi­cal instru­ment. Few peo­ple would argue that learn­ing to play the vio­lin is nat­ural; after all, it takes years of ded­i­ca­tion and hard work. A case can be made, in fact, that peo­ple are being max­i­mally human when they do things that con­tra­dict their biol­ogy. “Doing what comes nat­u­rally” is easy. It’s what non­hu­man ani­mals do. Per­haps only human beings can will them­selves to do things that go against their “nature.”

And finally, even though any­one aspir­ing to gen­uine monogamy will, on bal­ance, have to swim upstream against the cur­rent of his or her evo­lu­tion­ar­ily bequeathed incli­na­tions, there are also con­sid­er­able bio­log­i­cal forces sup­port­ing such efforts. Some ani­mals man­age to be monog­a­mous. Cal­i­for­nia mice (Per­omyscus cal­i­for­ni­cus), for exam­ple, pair up and remain paired, for­sak­ing all oth­ers, largely because of the pay­off derived from hav­ing two par­ents to care for off­spring. Beavers estab­lish last­ing pair-bonds that enable them to coop­er­ate in build­ing a valu­able, com­plex home site. The Mala­gasy giant jump­ing rat has evi­dently made the jump to monogamy because of the predator-fighting ben­e­fits thereby pro­vided. And among pygmy mar­mosets, monogamy gives males uncon­scious con­fi­dence of their pater­nity, which in turn sup­ports their incli­na­tion to be unusu­ally paternal.

And human beings? Our species ben­e­fits greatly from bi-parental care. We can profit from shared, rec­i­p­ro­cated effort, espe­cially when we’re con­fi­dent both part­ners will be around for the long term. In addi­tion, human beings are endowed with an array of hard-wired traits that can be used to strengthen monogamy, among them a pen­chant (per­haps even a need) to attach and con­nect so-called mir­ror neu­rons that under­lie empa­thy; hor­monal sys­tems, such as those involv­ing oxy­tocin and vaso­pressin, that relate sex­ual sat­is­fac­tion to pair-bonding; and neural plas­tic­ity that pro­motes the strength­en­ing of brain cir­cuits asso­ci­ated with repeated reward mech­a­nisms — includ­ing, in all like­li­hood, those acti­vated via inter­ac­tions with the same individual.

Add to this the fact that peo­ple have big brains, and hence, an abil­ity to res­cue monogamy from monot­ony, as well as the capac­ity to imag­ine the future and a vis­ceral dis­like of dis­hon­esty, and the effect of biol­ogy on monogamy becomes com­plex indeed. Not to men­tion the adap­tive sig­nif­i­cance of that thing called love.

To be sure, monogamy isn’t easy; nor is it for every­one. But any­one who claims that he or she sim­ply isn’t cut out for monogamy misses the point: No one is. At the same time, no one’s biol­ogy pre­cludes monogamy either.

As Jean-Paul Sartre famously advised (albeit in a dif­fer­ent con­text): “You are free; choose.”

David P. Barash, an evo­lu­tion­ary biol­o­gist, is a pro­fes­sor of psy­chol­ogy at the Uni­ver­sity of Wash­ing­ton. His most recent book — coau­thored with Judith Eve Lip­ton — is “Strange Bed­fel­lows: The Sur­pris­ing Con­nec­tion Between Sex, Evo­lu­tion and Monogamy.”



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