On the other hand, when people live within environments that encompass little stress and threats to the viability of offspring, the need for serious and committed relations is lowered and therefore promiscuity and infidelity are more common.
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.
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).
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 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.
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.