It’s in Your Genes
The good people at Scientific American tried to find a correlation between genetics and infidelity. They found that men who had a variant of the chemical vassopressin (which is dominant in voles) have a tendency to have problems of fidelity and in relationships in general.
Further research from the Binghamton University, State University of New York also did a study and found that people who had a variation of the chemical DRD4 had a tendency to engage in activities of infidelity. These people also tended to be more open to social situations and taking risks.