But smiles can be misleading and there are a few major exceptions to the rules that seem to govern marital success or failure.
"People who are narcissistic and a little bit hypomanic can be generally very happy. They're life-of-the-party people. But they're more focused on themselves and not so good at hearing others," Heitler said. "In one-on-one relationships, they can be difficult partners. It's all about them and their way."
Hertenstein thinks his research should just be taken as a general trend across populations.
"We just found a correlation," he said. "We know these two things are linked in some way or somehow."
Although he couldn't say how, Hertenstein was willing to give a few hypotheses.
"Maybe [people] who smile a lot attract happier people, or maybe people who are obedient to the photographer may be obedient to people in the relationship," he said.
The most surprising part of the study to Hertenstein was the predictive value of what psychologists often call the "thin slicing" -- or examining a small example of a behavior and then inferring characteristics or traits based on that slice.
"I do think it's pretty surprising that you could make a prediction with some accuracy over a number of people using a split second of their emotions," said Hertenstein.