Helen Fisher, a biological anthropologist at Rutgers University and author of Anatomy of Love: The Natural History of Monogamy, Adultery and Divorce, responds:
Several years ago I embarked on a project to see if the seven-year itch really exists. I began by studying worldwide data on marriage and divorce and noticed that although the median duration of marriage was seven years, of the couples who divorced, most did so around their fourth year together (the “mode”). I also found that divorce occurred most frequently among couples at the height of their reproductive and parenting years—for men, ages 25 to 29, and for women, ages 20 to 24 and 25 to 29—and among those with one dependent child.
To try to explain these findings, I began looking at patterns of pair bonding in birds and mammals. Although only about 3 percent of mammals form a monogamous bond to rear their young, about 90 percent of avian species team up. The reason: the individual that sits on the eggs until they hatch will starve unless fed by a mate. A few mammals are in the same predicament. Take the female fox: the vixen produces very thin milk and must feed her young almost constantly, so she relies on her partner to bring her food while she stays in the den to nurse.
But here's the key: although some species of birds and mammals bond for life, more often they stay together only long enough to rear their young through infancy and early toddlerhood. When juvenile robins fly away from the nest or maturing foxes leave the den for the last time, their parents part ways as well.
Humans retain traces of this natural reproductive pattern. In more contemporary hunter-gatherer societies, women tend to bear their children about four years apart. Moreover, in these societies after a child is weaned at around age four, the child often joins a playgroup and is cared for by older siblings and relatives. This care structure allows unhappy couples to break up and find a more suitable partner with whom to have more young.
In fact, serial pair bonding may have been beneficial to survival among our forebears because having children with more than one partner produces offspring with greater genetic variety and a wider range of skills. Hence, in the changeable environment of ancient Africa, some offspring would have had a better chance of enduring.
The four-year divorce peak among modern humans may represent the remains of an ancestral reproductive strategy to stay bonded at least long enough to raise a child through infancy and early toddlerhood. Thus, we may have a natural weak point in our unions. By understanding this susceptibility in our human nature, we might become better able to anticipate, and perhaps be able to avoid, the four-year itch.