Infidelity
Gender differences

There is currently debate in the field of evolutionary psychology whether an innate, evolved sex difference exists between men and women in response to an act of infidelity; this is often called a "sex difference". A study published in 2002 suggested there may be sex differences in jealousy. Those that posit a sex difference exists state that men are 60% more likely to be disturbed by an act of sexual infidelity (having one's partner engage in sexual relations with another), whereas women are 83% more likely to be disturbed by an act of emotional infidelity (having one's partner fall in love with another). Those against this model argue that there is no difference between men and women in their response to an act of infidelity. From an evolutionary perspective, men are theorized to maximize their fitness by investing as little as possible in their offspring and producing as many offspring as possible, due to the risk of males investing in children that are not theirs. Women, who do not face the risk of cuckoldry, are theorized to maximize their fitness by investing as much as possible in their offspring because they invest at least nine months of resources towards their offspring in pregnancy. Maximizing female fitness is theorized to require males in the relationship to invest all their resources in the offspring. These conflicting strategies are theorized to have resulted in selection of different jealousy mechanisms that are designed to enhance the fitness of the respective gender.

A common way to test whether an innate jealousy response exists between sexes is to use a forced-choice questionnaire. This style of questionnaire asks participants "yes or no" and "response A or response B" style questions about certain scenarios. For example, a question might ask, "If you found your partner cheating on you would you be more upset by (A) the sexual involvement or (B) the emotional involvement". Many studies using forced choice questionnaires have found statistically significant results supporting an innate sex difference between men and women. Furthermore, studies have shown that this observation holds across many cultures, although the magnitudes of the sex difference vary within sexes across cultures.

Although forced-choice questionnaires show a statistically significant sex-difference, critics of the theory of evolved sex differences in jealousy question these findings. In consideration of the entire body of work on sex differences, C. F. Harris asserted that when methods other than forced-choice questionnaires are used to identify an innate sex difference, inconsistencies between studies begin to arise. For example, researchers found that women sometimes report feeling more intense jealousy in response to both sexual and emotional infidelity. The results of these studies also depended on the context in which the participants were made to describe what type of jealousy they felt, as well as the intensity of their jealousy.

In her meta-analysis, Harris raises the question of whether forced choice questionnaires actually measure what they purport: jealousy itself and evidence that differences in jealousy arise from innate mechanisms. Her meta-analysis reveals that sex-differences are almost exclusively found in forced-choice studies. According to Harris, a meta-analysis of multiple types of studies should indicate a convergence of evidence and multiple operationalizations. This is not the case, which raises the question as to the validity of forced-choice studies. DeSteno and Bartlett (2002) further support this argument by providing evidence which indicates that significant results of forced-choice studies may actually be an artifact of measurement; this finding would invalidate many of the claims made by those "in favor" of an "innate" sex difference. Even those "in favor" of sex-differences admit that certain lines of research, such as homicide studies, suggest against the possibility of sex-differences.

These inconsistent results have led researchers to propose novel theories that attempt to explain the sex differences observed in certain studies. One theory that has been hypothesized to explain why men and women both report more distress to emotional infidelity than sexual infidelity is borrowed from childhood attachment theories. Studies have found that attachment styles of adults are consistent with their self-reported relationship histories. For example, more men are reported to have an insecure, dismissing avoidant attachment style; where these "individuals often attempt to minimize or constrict emotional experience, deny needs for intimacy, are highly invested in autonomy, and are more sexually promiscuous than individuals who have other attachment styles". Levy and Kelly (2010) tested this theory and found that adult attachment styles strongly correlate to which type of infidelity elicited more jealousy. Individuals who have secure attachment styles often report that emotional infidelity is more upsetting whereas dismissing attachment styles were more likely to find sexual infidelity more upsetting. Their study did report that men in general were more likely than women to report sexual infidelity as more distressing, however this could be related to more men having a dismissing attachment style.The authors propose that a social mechanism may be responsible for the observed results. In other words, replicable sex differences in emotion and sexual jealousy could be a function of a social function. Similar studies focusing on the masculinization and feminization by society also argue for a social explanation, while discounting an evolutionary explanation.

A 2015 study found a correlation between AVPR1A expression and predisposition to extrapair mating in women but not in men.
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