Headed to the movies this weekend? Here's a thought to ponder: Of the 39 major studio releases scheduled for this summer, only one is directed by a woman -- that would be "Jupiter Down," co-directed by Lana Wachowski and her brother Andy. That's according to a recent analysis by the Wrap, a Hollywood news site.

When you think of gender in Hollywood, you probably think of the issues facing actresses -- the disparities in screen time for men and women, the lack of strong roles for women, the continuing failure of films to pass the Bechdel test. But many of these issues arise from what's happening behind the camera -- namely, the astonishing lack of women in directorial roles, particularly on big-budget films.

I wanted to quantify this, so I gathered data on the top 400 films by domestic box-office gross per year, from 2009 to 2013, from the film data site the-numbers.com. I supplemented this data with numbers from the Rotten Tomatoes API, particularly to fill in missing director names. I then ran the directors' first names through genderize.io to obtain the likely gender of each director, supplementing this with some manual entry when genderize couldn't produce a result with high certainty. All in all, I ended up with a dataset of about 2,000 films and their directors. Some key findings on the state of gender equality in the director's chair are below.

Women are directing more films overall. But female directors are still rare at the top of the box office.

In 2013, women directed about 14 percent of the films in the-numbers.com's database of top box office draws. This is up from a low of about 8 percent in 2010. But women are much more scarce at the top of the box office. They directed only two of the top 100 films in 2013 -- "Frozen" and the remake of "Carrie." Female directors are more poorly represented among Hollywood's biggest films than they are in the executive suite at Google.

Most observers agree that the lack of movies directed by women isn't necessarily due to a shortage of female directors. A vicious cycle of financing is the primary reason -- studios and producers are hesitant to hand big budgets to female directors without proven track records. But if they can't get funding, they can't get a track record to begin with. There's no doubt that good old-fashioned sexism also plays a large role here.

Documentary, romantic comedy and drama are the genres most likely to be directed by women.

Nearly a quarter of documentary releases from 2009 to 2013 were directed by women. Romantic comedies were a distant second, at 16 percent. Everyone thinks of "Bridesmaids" as the quintessential "female" comedy -- yet that movie doesn't even show up in these figures, since it was directed by a man (not that there's anything wrong with that -- some of my best friends are men).

At the opposite end of the list were action and adventure movies. This partially explains why we see so few women behind the cameras of this summer's blockbuster releases.

On the release calendar, women-made films peak in May and September.

Only about 7 percent of films released in January and February are helmed by women. By contrast, this share doubles to more than 15 percent in May and September. The seasonality here appears to be most closely tied to the release schedule of romantic comedies, which also peak in May and September.

Overall, the argument here isn't just a moral one -- it's a financial one too. More than half of moviegoers are now women, according to the MPAA. And it's a fair bet that many of those women, not to mention plenty of men, would be happy to see industry's idea of a "summer blockbuster" expanded beyond its current Bruckheimerian ideal of a buxom woman draped across a Trans Am while giant robots shoot missiles at each other in the background. Putting more women in the director's chair would be a good first step.