Marital discord has a genetic component in men

Men are more likely to be devoted and loyal husbands when they lack a particular variant of a gene that influences brain activity...which is present in two of every five men...[it] also appears to predict whether women involved with these men are likely to say their partners are emotionally close and available, or distant and disagreeable. The presence of the gene variant, or allele, also seems predictive of whether men get married or live with women without getting married. 

Obviously, it's not just genes that determine marital fidelity and harmony, but it's fascinating to see how science continues to discover the role of nature in complex relationships.