It's a finding that could help justify heavy spending on makeup and wardrobe for Republican vice presidential nominee, while at the same time raising questions about the need for a man like John Edwards to invest in a costly haircut.
"What we found was quite startling," said Joan Y. Chiao of Northwestern University's psychology department.
For male candidates, the only thing that mattered was competence, while female voters preferred men who seemed both competent and approachable.
But for "female candidates for a hypothetical election for the United States presidency, both male and female voters were more likely to vote for candidates that were both competent and attractive," Chiao said in a telephone interview.
"Neither trait (alone) was sufficient to predict whether a person was going to vote for that candidate," she added. Chiao's findings are being published online by the journal PLoS ONE.
"For female candidates, it really matters if they're perceived as competent and perceived as attractive. Those two qualities are sort of twin predictors of whether or not someone is going to be more or less likely to vote for them," Chiao stressed.
"There are a lot of potential theories," she said. Most likely may be the way people choose friends and mates.
"There's a lot of talk about voters thinking, in their mind, 'Who would I like to go out for a beer with' when they're evaluating potential candidates. We think that that taps really well into the gut instincts that voters use when they're thinking about who they're going to vote for."
"These gut instincts that we use in mate selection are operating unconsciously in leader selection," she explained. "This is all operating unconsciously, people don't even realize how they're making these decisions," she said.
Past studies have also looked at the effect of attractiveness in politics.
A 2005 study, for example, found that candidates who appeared more mature did better than ones with a babyface. Researchers at that time said maturity helped voters infer competence in the candidate.
So, how did Chiao and her colleagues reach their conclusion?
They collected photos of congressional candidates from 2006 and asked a panel of 73 college students to rate the candidates for competence, dominance, attractiveness and approachability.
None of the students recognized any of the candidates and most had never voted in a real election.
Overall, men tended to be rated more competent than women. Female students rated male candidates as appearing more dominant, while male students saw no difference in dominance.
All the students rated female candidates more attractive than men, and female candidates were rated as more approachable.
Once they had the sets of ratings for the candidates, the researchers divided them into pairs and asked the students to select which one they would be more likely to choose as president.
Their conclusions about the qualities needed to win votes were based on the winners of those hypothetical contests.
So, would these results be the same for adults?
"It's possible older adults, past the stage of romantic partnerships, may show less of a gender bias," Chiao said.
And she said familiarity seems to breed comfort. As more and more women win office, voters tend to become more comfortable choosing them.