This study Oxytocin Modulates Social Distance between Males and Females examines the effects of Oxytocin (OXT) on men in monogamous relationships. It demonstrates that OXT assists in maintaining fidelity to that relationship. OXT levels are increased by close and sexual interactions with their partner.

The specificity of the OXT effect in influencing the social distance that males keep from unfamiliar females is evidenced by our finding that it has no significant effect on the distance kept from unfamiliar males. Given the absence of PS differences between single and pair-bonded men in the PLC group, it is clear that for these potential fidelity-enhancing effects of OXT to be revealed, female partners would need to evoke its endogenous release immediately before contexts in which the men might encounter other women. While, compared with singles, basal concentrations of OXT in blood are increased in couples during the early stages of romantic love and stay significantly elevated in couples remaining together 6 months later (Schneiderman et al., 2012), our results suggest that further augmentation of endogenous OXT release is necessary to produce fidelity-enhancing effects. Mechanistically, this may be related to additional OXT either compensating a relative deficit (optimization hypothesis) or by collapsing already optimal levels (decompensation hypothesis). While the most obvious physiological stimulus for promoting endogenous OXT release in men would be having sex with their mate (Kru¨ger et al., 2003), the simple close presence and touch of their partner at any given moment in time might also suffice (Holt-Lunstad et al., 2008). Thus, OXT effects in promoting monogamy in males may normally depend upon the presence of a close positive relationship in the bond with their female partners and close physical proximity between the couple.1

There have been studies on the factors involved in sexual infidelity.

Virtually all American couples, married or cohabiting, expect sexual exclusivity of one another. This article asks why some people are sexually exclusive while others have sex with someone besides their mate. Previous research has linked personal values, sexual opportunities, and quality of the marital relationship to extramartial sex. This paper integrates these findings in a multivariate model that incorporates factors informing sexual decision making as well as demographic “risk factors.” Nationally representative survey data show higher likelihood of sexual infidelity among those with stronger sexual interests, more permissive sexual values, lower subjective satisfaction with their union, weaker network ties to partner, and greater sexual opportunities. With these factors controlled, gender differences are substantially reduced or eliminated, although racial effects persist. 2

Given that there are people who can maintain close relationships, simultaneously, with more than one partner.
Have there been studies examining the brain biochemistry that increases an individual's propensity for sexual infidelity?

1 Oxytocin Modulates Social Distance between Males and Females Dirk Scheele1, Nadine Striepens1, Onur Güntürkün, Sandra Deutschländer, Wolfgang Maier1, Keith M. Kendrick, and René Hurlemann The Journal of Neuroscience, 14 November 2012, 32(46): 16074-16079; doi: 10.1523/​JNEUROSCI.2755-12.2012

2 Sexual Infidelity Among Married and Cohabiting Americans Judith Treas1, Deirdre Giesen2 Article first published online: 2 MAR 2004 DOI: 10.1111/j.1741-3737.2000.00048.

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    $\begingroup$ One thought: People differ in what they perceive as infidelity, so if a researcher measures all people against the same standard, many will have committed acts of infidelity that they themselves and their spouses did not perceive as infidelity (and vice versa). See the common and controversial forum discussions on whether imagining having sex is already infidelity, or statements that one night stands are not yet a breach of trust. [contd.] $\endgroup$ – user3116 Aug 21 '13 at 14:13
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    $\begingroup$ [contd.] And I wouldn't simply define these away as misconceptions, because the same behavior can cause quite different reactions and emotions in different people: some people will end a relationship because their partner constantly flirts with others; while some people remain committed even after they discovered their partner had a lengthy affair. What defines infidelity? Certainly not the researcher. $\endgroup$ – user3116 Aug 21 '13 at 14:13
  • $\begingroup$ No, it is not an answer, neither to this nor any other question. I just have issues with these sort of studies. The paper says OXT modulates fidelity. Now, OXT is, for example, produced through sex. But if the man and woman simply don't have sex, then it is not the absence of OXT that makes men be unfaithful, but something else that comes before the chance to release OXT. OXT is not something that makes men faithful, but something that makes them feel close to the person that they are faithful to. The faithfulness comes before the OXT. $\endgroup$ – user3116 Aug 21 '13 at 16:34

The number one hormonal suspect in infidelity has been testosterone. Partially this has been predicted because testosterone predicts sex drive in both men and women. The problem with testing this one in actual humans though is that most of the things associated with cheating also cause increased testosterone levels (flirting, increased number of sexual partners). Therefore, there's a serious chicken and egg problem in more observational studies.

Roney, J. R., Simmons, Z. L., & Lukaszewski, A. W. (2010). Androgen receptor gene sequence and basal cortisol concentrations predict men’s hormonal responses to potential mates. Proceedings of the Royal Society of London B, 277, 57-63.


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