Although people talk about seven years, divorce rates have historically peaked at around four years (Fisher, 1989). Biological anthropologist Helen Fisher argues that this four-year peak makes sense from an evolutionary perspective.
In the course of human evolution, women who changed partners after four years together (enough time to co-parent through the early hard years of having a couple of kids) may have had an adaptive advantage. By engaging in “serial pair-bonding,” they could vary the genetic make-up of their offspring. The timing of today’s peaks in divorce rates may reflect the ingrained drive towards variation.
More recent research (Kulu, 2014) suggests that divorce rates rise after marriage and then peak at about five years. Rates of divorce then steadily decline as years together increase. This rising-falling pattern is reminiscent of the seven-year-itch argument but occurs slightly earlier (a five-year itch?) than the phrase suggests.