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.For example, one study found that some women in more financially independent and higher positions of power, were also more likely to be more unfaithful to their partners.
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.
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.
According to The New York Times, the most consistent data on infidelity comes from the University of Chicago's General Social Survey (GSS).
Interviews with people in non-monogamous relationships since 1972 by the GSS have shown that approximately 12% of men and 7% of women admit to having had an extramarital relationship.
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 so this type of behaviour, which is desired by men, is more accepted.
On the other hand, when sex ratios are low, promiscuity is less common because women are in demand and since they desire monogamy and commitment, in order for men to remain competitive in the pool of mates, they must respond to these desires.
Strategic pluralism is a theory that focuses on how environmental factors influence mating strategies.
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.
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%.