Attention political parties: want to get more votes? Run more women for office. Science says so.

That’s the finding of a recent study co-authored by MIT economist Albert Saiz and Pablo Casas-Arce, an assistant professor of economics at Arizona State University. Saiz and Casas-Arce examined the consequences of a recent legislative change in Spain: In 2004, a law was passed requiring political parties to have women comprise at least 40 percent of their candidates for local offices. The law was very effective at accomplishing its goal of getting more women candidates: the number of female candidates increased by 32 percent during the first election cycle under the law. But even more importantly? The political parties that upped their share of women candidates did better than their opponents.

Parties that increased their share of women candidates by 10 percent more than their opposition did better at the ballot box by an average of 4.2 percentage points. As Saiz points out, forcing parties to field more female candidates doesn’t force parties to make electoral concessions in the interests of gender equality: it does the opposite and helps those parties win elections over parties that don’t field as many women on their slates.

What’s the upshot?

The implementation of the law in Spain provided an excellent test case, but its scientific relevance is limited: unlike their equivalents in the United States, political parties in Spain get to choose the list of candidates they field for municipal offices, rather than candidates running by themselves and associating with a particular party if they choose. In addition, attitudes of voters in Spain may be different from those elsewhere in the world. But according to Saiz, this study suggests that at least in Spain, voters have an appetite for female candidates, and there are plenty of qualified female candidates to choose from—but in the past they haven’t made the list because of the internal struggles within political parties. As he says, “there’s some elbowing out going on that leaves women behind.”

Regardless of the scope, though, it’s a strong piece of evidence against the idea that voters aren’t interested in female candidates or that there aren’t enough qualified women to run for office. And given the fact that fewer than 20 percent of all seats in Congress are held by women, this study ought to revitalize the discussion of why women don’t make up an equal share of lawmakers here at home. If it’s not the voters and it’s not the candidates, there must be something wrong with the system.