Reported in LiveScience:
In one test, the researchers looked at people's college yearbook photos, and rated their smile intensity from 1 to 10. None of the people who fell within the top 10 percent of smile strength had divorced, while within the bottom 10 percent of smilers, almost one in four had had a marriage that ended, the researchers say. (Scoring was based on the stretch in two muscles: one that pulls up on the mouth, and one that creates wrinkles around the eyes.)
In a second trial, the research team asked people over age 65 to provide photos from their childhood (the average age in the pictures was 10 years old). The researchers scored each person's smile, and found that only 11 percent of the biggest smilers had been divorced, while 31 percent of the frowners had experienced a broken marriage.
Abstract: Motivation and Emotion, April 2009