helen_fisher's picture
Biological Anthropologist, Rutgers University; Author, Why Him? Why Her? How to Find and Keep Lasting Love
Planned Obsolescence? The Four-Year Itch

When asked why all of her marriages failed, anthropologist Margaret Mead apparently replied, "I beg your pardon, I have had three marriages and none of them was a failure."  There are many people like Mead.  Some 90% of Americans marry by middle age.  And when I looked at United Nations data on 97 other societies, I found that more than 90% of men and women eventually wed in the vast majority of these cultures, too.  Moreover, most human beings around the world marry one person at a time: monogamy.  Yet, almost everywhere people have devised social or legal means to untie the knot.  And where they can divorce — and remarry — many do. 

So I had long suspected this human habit of "serial monogamy" had evolved for some biological purpose.  Planned obsolescence of the pairbond?  Perhaps the mythological "seven-year itch" evolved millions of years ago to enable a bonded pair to rear two children through infancy together.  If each departed after about seven years to seek "fresh features," as poet Lord Byron put it, both would have ostensibly reproduced themselves and both could breed again — creating more genetic variety in their young. 

So I began to cull divorce data on 58 societies collected since 1947 by the Statistical Office of the United Nations.  My mission: to prove that the "seven year itch" was a worldwide biological phenomenon associated in some way with rearing young.  

Not to be.  My intellectual transformation came while I was pouring over these divorce statistics in a rambling cottage, a shack really, on the Massachusetts coast one August morning.  I regularly got up around 5:30, went to a tiny desk that overlooked the deep woods, and poured over the pages I had Xeroxed from the United Nations Demographic Yearbooks.  But in country after country, and decade after decade, divorces tended to peak (the divorce mode) during and around the fourth year of marriage.  There were variations, of course.  Americans tended to divorce between the second and third year of marriage, for example.  Interestingly, this corresponds with the normal duration of intense, early stage, romantic love — often about 18 months to 3 years.  Indeed, in a 2007 Harris poll, 47% of American respondents said they would depart an unhappy marriage when the romance wore off, unless they had conceived a child.

Nevertheless, there was no denying it:  Among these hundreds of millions of people from vastly different cultures, three patterns kept emerging.  Divorces regularly peaked during and around the fourth year after wedding.  Divorces peaked among couples in their late twenties.  And the more children a couple had, the less likely they were to divorce: some 39% of worldwide divorces occurred among couples with no dependent children; 26% occurred among those with one child; 19% occurred among couples with two children; and 7% of divorces occurred among couples with three young. 

I was so disappointed.  I mulled about this endlessly.  My friend used to wave his hand over my face, saying, "Earth to Helen; earth to Helen."  Why do so many men and women divorce during and around the 4-year mark; at the height of their reproductive years; and often with a single child?  It seemed like such an unstable reproductive strategy.  Then suddenly I got that "ah-ha" moment:  Women in hunting and gathering societies breastfeed around the clock, eat a low-fat diet and get a lot of exercise — habits that tend to inhibit ovulation.  As a result, they regularly space their children about four years apart.  Thus, the modern duration of many marriages—about four years—conforms to the traditional period of human birth spacing, four years. 

Perhaps human parental bonds originally evolved to last only long enough to raise a single child through infancy, about four years, unless a second infant was conceived.  By age five, a youngster could be reared by mother and a host of relatives. Equally important, both parents could choose a new partner and bear more varied young.

My new theory fit nicely with data on other species.  Only about three percent of mammals form a pairbond to rear their young.  Take foxes.  The vixen's milk is low in fat and protein; she must feed her kits constantly; and she will starve unless the dog fox brings her food.  So foxes pair in February and rear their young together.  But when the kits leave the den in mid summer, the pairbond breaks up.  Among foxes, the partnership lasts only through the breeding season.  This pattern is common in birds.  Among the more than 8,000 avian species, some 90% form a pairbond to rear their young.  But most do not pair for life. A male and female robin, for example, form a bond in the early spring and rear one or more broods together.  But when the last of the fledgling fly away, the pairbond breaks up.  

Like pair-bonding in many other creatures, humans have probably inherited a tendency to love and love again—to create more genetic variety in our young.  We aren't puppets on a string of DNA, of course.  Today some 57% of American marriages last for life.  But deep in the human spirit is a restlessness in long relationships, born of a time long gone, as poet John Dryden put it, "when wild in wood the noble savage ran."