Some psychological traits may make people more likely to be infidels
Scientists find predictors for infidelity and divorce - short science news

A study that followed 223 newly married couples provides insights into the psychological traits that lead to infidelity. The findings show that participants who quickly disengaged their attention from an attractive person (other than their partner) were less likely to engage in infidelity. On the contrary, partners who took significantly longer to look away from romantic alternatives had a higher risk of infidelity, and their marriages were more likely to fail. These are automatic psychologically-driven reactions and researchers argue they could be predictors of the success of a relationship. 

Read the full story: Florida State University
Scientific publication: Journal of Personality and Social Psychology