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 this type of behaviour, 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.
Infidelity (synonyms include: cheating, adultery (when married), netorare (NTR), being unfaithful, or having an affair) is a violation of a couple's assumed or stated contract regarding emotional and/or sexual exclusivity.
Other scholars define infidelity as a violation according to the subjective feeling that one's partner has violated a set of rules or relationship norms; this violation results in feelings of sexual jealousy and rivalry.
People who had stronger sexual interests, more permissive sexual values, lower subjective satisfaction with their partner, weaker network ties to their partner, and greater sexual opportunities were more likely to be unfaithful.Correspondingly, monogamy and commitment are more commonplace.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.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.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 parent.