Why do so few women get Best Director nominations or wins?

At the Golden Globes this year, not a single woman was nominated for Best Director. Women were shut out of the category last year, too. Very few women directors have ever been nominated. Only one has ever won: Barbra Streisand 35 years ago. A new USC Annenberg study found that of the 100 top grossing films of last year, less than 4 percent were directed by women.