These differences have been generally thought due to evolutionary pressures that motivate men towards sexual opportunity and women towards commitment to one partner.
These findings suggest there may be various factors that might influence the likelihood of some individuals to engage in extradyadic relationships, and that such factors may account for observed gender differences beyond actual gender and evolutionary pressures associated with each.
Results, however, vary year by year, and also by age-group surveyed.
For example, one study conducted by the University of Washington, Seattle found slightly, or significantly higher rates of infidelity for populations under 35, or older than 60.
One measure of infidelity is covert illegitimacy, a situation which arises when someone who is presumed to be a child's father (or mother) is in fact not the biological father (or mother).
Frequencies as high as 30% are sometimes assumed in the media, but research Such studies show that covert illegitimacy is in fact less than 10% among the sampled African populations, less than 5% among the sampled Native American and Polynesian populations, less than 2% of the sampled Middle Eastern population, and generally 1–2% among European samples.