Some interesting new research bears on this question:
On average, women make up about 20 percent of lawmakers in the United States and abroad. We found that when women constituted 20 percent of a decision-making body that operates by majority rule, the average woman took up only about 60 percent of the floor time used by the average man. Women were perceived — by themselves and their peers — as more quiescent and less effective. They were more likely to be rudely interrupted; they were less likely to strongly advocate their policy preferences; and they seldom mentioned the vulnerable. These gender dynamics held even when adjusting for political ideology (beliefs about liberalism and egalitarianism) and income. In contrast, the men in our experiment did not speak up less or appear to lose influence when they were in the minority.
[Cross-posted at The Monkey Cage]