People who had stronger sexual interests, more permissive sexual values, lower subjective satisfaction with their partner, weaker network ties to their partner, and greater sexual opportunities were more likely to be unfaithful.
Studies suggest around 30–40% of unmarried relationships and 18–20% of marriages see at least one incident of sexual infidelity.
A survey conducted in 1990 found 2.2% of married participants reported having more than one partner during the past year.
Results, however, vary year by year, and also by age-group surveyed.
According to this theory, when people live within environments that are demanding and stressful, the need for bi-parental care is greater for increasing the survival of offspring.
Correspondingly, monogamy and commitment are more commonplace.
Rates of infidelity among women are thought to increase with age.
In one study, rates were higher in more recent marriages, compared with previous generations; men were found to be only "somewhat" more likely than women to engage in infidelity, with rates for both sexes becoming increasingly similar.