A series of unusual studies led by John Lydon, a psychologist at McGill University in Montreal, have looked at how people in a committed relationship react in the face of temptation. In one study, highly committed married men and women were asked to rate the attractiveness of people of the opposite sex in a series of photos. Not surprisingly, they gave the highest ratings to people who would typically be viewed as attractive.

Later, they were shown similar pictures and told that the person was interested in meeting them. In that situation, participants consistently gave those pictures lower scores than they had the first time around.

When they were attracted to someone who might threaten the relationship, they seemed to instinctively tell themselves, “He’s not so great.” “The more committed you are,” Dr. Lydon said, “the less attractive you find other people who threaten your relationship.”