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.
Another study found that the likelihood for women to be involved in infidelity reached a peak in the seventh year of their marriage and then declined afterwards; whereas for married men, the longer they were in relationships, the less likely they were to engage in infidelity, except for the eighteenth year of marriage, at which point the chance that men will engage in infidelity increases.
In terms of infidelity, the theory states that when sex-ratios are high, men are more likely to be promiscuous and engage in sex outside of a committed relationship because the demand for men is higher and this type of behaviour, desired by men, is more accepted.
Differences in sexual infidelity as a function of gender have been commonly reported.
Studies suggest around 30–40% of unmarried relationships and 18–20% of marriages see at least one incident of sexual infidelity.
Rates of infidelity among women are thought to increase with age.
It is more common for men compared to women to engage in extradyadic relationships.
The National Health and Social Life Survey found that 4% of married men, 16% of cohabiting men, and 37% of dating men engaged in acts of sexual infidelity compared to 1% of married women, 8% of cohabiting women, and 17% of women in dating relationships.