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.
Correspondingly, monogamy and commitment are more commonplace.
A survey conducted in 1990 found 2.2% of married participants reported having more than one partner during the past year.
In general, national surveys conducted in the early 1990s reported that between 15-25% of married Americans reported having extramarital affairs.
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.
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.
When they are not met, research has found that psychological damage can occur, including feelings of rage and betrayal, lowering of sexual and personal confidence, and damage to self-image.
Depending on the context, men and women can experience social consequences if their act of infidelity becomes public.
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.
Such covertly illegitimate children amount to about 1–2% of newborns in European populations.