“We found that broad smiles lead people to be perceived as warmer but less competent,” said Jessica Li, a KU assistant professor of marketing in the School of Business. “We ask how that can influence consumer behavior and in what situations might marketers want to smile more broadly.”
[…]
Li said broader smiles that tend to elicit more warmth seem to be more effective in promotional ads for a service that would carry less risk. But photos with a slight smile did better in marketing scenarios where services were higher risk, such as a medical procedure, legal representation or investment in a startup company.

“If I see an ad with a heart surgeon who smiles really broadly at me, I might think she is really warm, but not choose her to be my doctor because she seems less competent than a surgeon with a slight smile,” Li said. “If the risk is really low, such as going to the store to get a new shirt, then the competence of the salesperson isn’t as important and I respond more positively to the broad smile.”

In their analysis of Kickstarter.com, when the page creator’s profile photo exhibited a broad smile that tended to elicit perceptions of warmth, the total amount of money pledged decreased by more than 50 percent, and the average contribution per backer was 30 percent less than when the creator’s photo included only a slight smile.

“Project creators with a slight smile are perceived as more competent,” Li said. “More people wanted to donate to their project because they believe this competent person is able to deliver the product.”

However, a more intense smile does appear to elicit more buzz on social media or other low-cost behaviors. Profile photos with a broader smile received twice as many Facebook shares than someone with a slight smile.

Source: Don’t smile too big to be effective in online marketing ads, study funds