His fear of commitment may lie in his "genes" after all, a study finds.
Researchers say they've found a genetic variation that may be responsible for weakening some men's ability to be monogamous, Science News reports.
The study, to appear in the medical journal Proceedings of the National Academy of Sciences, is the first to examine whether a hormone that encourages monogamy in animals plays a similar role in male humans, the magazine reports.
Scientists studied gene variations in about 500 men who are in committed relationships. In addition to the gene study, the men and their mates, who had been together at least five years, answered questions about their relationship, including whether they had ever considered a divorce and how often they kissed their spouses.
Looking at a gene that controls a brain receptor of the hormone vasopressin, researchers found that some men who had a gene variation, called allele 334, bonded less with their mates and had more relationship conflict, according to the report.
Vasopressin has been found to play a role in whether certain animals remain monogamous.
Fifteen to 16 percent of the men who had either no copies or just one copy of the 334 allele reported a marital crisis in the past year, Science News reported.
But 34 percent of men who had two copies of the 334 allele reported conflict. Despite the findings, scientists believe it's a combination of behaviors of the individuals involved in a relationship and genes that make or break couples.