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.
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.
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.
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.
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.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.Support for this theory comes from evidence showing higher divorce rates in countries with lower sex ratios and higher monogamy rates in countries with higher sex ratios.Differences in sexual infidelity as a function of gender have been commonly reported.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.