Studies linking brain chemistry to sexual infidelity
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., &amp; 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.