Sex ratio theory is a theory that explains the relationship and sexual dynamics within different areas of the world based on the ratio of the number of marriage-aged men to marriage-aged women.
According to this theory, an area has a high sex ratio when there is a higher number of marriage-aged women to marriage-aged men and an area has a low sex ratio when there is more marriage-aged men to marriage-aged women.
In one study by Blow, rates were higher in more recent marriages, compared with previous generations.
A study by Liu 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 are in relationships the less likely they are to engage in infidelity, except for the eighteenth year of marriage, at which point the chance that men will engage in infidelity increases.
Such covertly illegitimate children amount to about 1–2% of newborns in studied populations.
These differences have been generally thought due to evolutionary pressures that motivate men towards sexual opportunity and women towards commitment to one partner.
In addition, recent research finds that differences in gender may possibly be explained by other mechanisms including power and sensations seeking.
In that study which involved 19,065 people during a 15-year period, rates of infidelity among men were found to have risen from 20 to 28%, and rates for women, 5% to 15%.
A survey conducted in 1990 found 2.2% of married participants reported having more than one partner during the past year.