I do think that Matt Hertenstein and colleagues came up with an eye catching title here:

Hertenstein, M., C. Hansel, et al. (2009). "Smile intensity in photographs predicts divorce later in life." Motivation and Emotion 33(2): 99-105.  [Abstract/Full Text]  [Free Full Text]
Abstract: Based on social-functional accounts of emotion, we conducted two studies examining whether the degree to which people smiled in photographs predicts the likelihood of divorce. Along with other theorists, we posited that smiling behavior in photographs is potentially indicative of underlying emotional dispositions that have direct and indirect life consequences. In the first study, we examined participants' positive expressive behavior in college yearbook photos and in Study 2 we examined a variety of participants' photos from childhood through early adulthood. In both studies, divorce was predicted by the degree to which subjects smiled in their photos.

Obviously there are many other factors also involved in whether someone will get divorced or not!  However the effects were pretty strong with the 10% with most intense smiles only having a fifth the chance of getting divorced compared with the least smiling 10%.  Other research has pointed in the same sort of direction, so Sonja Lyubomirsky and colleagues have argued convincingly that greater happiness is not only caused partly by doing well in different areas of one's life, it is also a cause of doing well in different life areas - including relationships.  There's also the "associative mating" argument - happy people tend to marry other happy people.  There's the finding that, when interpersonal difficulties do arise, optimists seem more able to sort things out constructively - seeing things positively can often make them more likely to be so.  And even research showing that emotions are "infectious".

Actually Hertenstein's main research work is more on touch than on smiling.  See his "Touch and emotion lab" with its useful series of fascinating freely downloadable full text research studies including one I particularly like entitled "The communicative functions of touch in humans, non-human primates and rats: A review and synthesis of the empirical research".

Since first writing this blog post a further fascinating research study has emerged:

Abel, E. L. and M. L. Kruger (2010). "Smile intensity in photographs predicts longevity." Psychol Sci 21(4): 542-544.  [PubMed] 
(As described in the BPS Research Digest at http://www.bps-research-digest.blogspot.com/ on 11/6/10) Look at a person's photo and it's tempting to think you can see their personality written all over it: stony-faced individuals appear somber; others flashing a big, toothy grin seem more genial. An intriguing new study claims that these smiles are a reliable marker of underlying positive emotion and as such are predictive of a person's longevity. Ernest Abel and Michael Kruger had five people rate the smile intensity of 230 baseball players according to photos featured in the 1952 Baseball Register. The researchers used a three-point smile scale: no smile, half smile (mouth only), and genuine 'Duchenne' smile (muscles contracted around the mouth and corners of the eyes). Focusing on the 150 players who'd died by the time of the study and controlling for extraneous factors such as BMI and marital status, the researchers found that those who were flashing a genuine 'Duchenne Smile' were half as likely to die in any given year compared with non-smilers. Indeed, the average life-span of the 63 deceased non-smilers was 72.9 years compared with 75 years for the 64 partial smilers and 79.9 years for the 23 Duchenne smilers. A follow-up study was similar to the first but observers rated the attractiveness of the same players rather than their smile intensity. Unlike smile intensity, attractiveness bore no relation to longevity. 'To the extent that smile intensity reflects an underlying emotional disposition, the results of this study are congruent with those of other studies demonstrating that emotions have a positive relationship with mental health, physical health, and longevity,' the researchers said.