Men will often ask themselves, “What do women want, a bad boy or a good guy?” Although the bad boy is well-known to make women weak in the knees, the good guy has a lot more to give with his friendly face. According to a recent study published in the journal Evolutionary Psychology, smiling men are more apt for a long-term relationship because they appear to be more trustworthy.

Evolutionary psychologists have long theorized humans have two distinct basic strategies for selecting a partner. When it comes to short-term relationships, people focus on finding a partner with the best genes — one who could possibly produce offspring. For long-term relationships, however, partners focus not only on finding someone with good genes, but someone who also possesses the paternal capabilities to raise children. Biologically speaking, this is why the qualities people deem attractive are contingent on whether they see their potential mate as a short-term or long-term prospect.

A 2011 study published in the journal Emotion found men saw women as more attractive when they smiled, and least attractive when they looked proud or confident. However, there has been inconclusive research on the opposite — do women find smiling men more attractive?

Researchers from the University of Oslo and Senshu University in Japan sought to determine how women perceive men in the context of the type of relationship they were looking for. A total of 218 female university students from Japan, Norway, and Italy were recruited to rate the attractiveness of a series of male faces that were either smiling or had a neutral expression. Next, they were asked to rate how willing they would be to date the men in a short-term relationship or commit to a long-term relationship, with the possibility of marriage. At the end of it all, smiling men were seen as better potential long-term partners, while they had no effect on women’s potential short-term partners.

For the second half of the study, 71 female Japanese university students rated the same smiling and neutral faces to see which qualities were deemed most suitable for a specific type of relationship. The women rated smiling men as less masculine and less mature, but more trustworthy, when compared to the men who had neutral facial expressions. This could be traced back to our evolutionary instinct to find a partner who possesses traits that boost the odds of survival.

"These results are in agreement with the idea that social and cooperative characteristics would be primarily important for long-term partners but not very much for short-term partners because long-term cooperation is necessary for parenting in the former but not in the latter," wrote the researchers. In other words, happiness and trustworthiness matter in the long term, but for the short term, women couldn't care less about some flaws.

A recent study presented at the 65th Annual Conference of the International Communication Association in San Juan, Puerto Rico found men’s appearances in online dating profile photos also affected women’s perceptions of trustworthiness. Women were more likely to find a guy more attractive and trustworthy when their photos were enhanced.

So men, if you’re looking for love, crack a smile and change your profile photo; you’ll appear more attractive and trustworthy.

Sources: Okubo M, Ishikawa K, Kobayashi A et al. Cool Guys and Warm Husbands: The Effect of Smiling on Male Facial Attractiveness for Short- and Long-Term Relationships. Evolutionary Psychology . 2015.

Tracy JL and Beall AT. Happy guys finish last: The impact of emotion expressions on sexual attraction. Emotion . 2011.

International Communication Association. Enhanced dating site photos have mixed results for men and women. 2015.