The scope of the disparity varies from study to study, but the general pattern remains the same: Men are more likely to interrupt other people in general, and they interrupt women more often than they interrupt other men. And women, when they make interruptions, are also much more likely to interrupt other women than they are to interrupt men.
This pattern harms women in many ways, particularly when it comes to career advancement. For most women in the workplace, the phenomenon is exhaustingly familiar: A woman offers an idea in a meeting, but nobody notices or acknowledges it until a man later says the same thing. When women are interrupted more often, they are heard less. And sometimes they are interrupted more often because they are already heard less.
It's true that interruptions are a complicated subject in sociolinguistics. Gender isn't the only factor that determines patterns of interruption; an individual's social status or cultural upbringing also matters a lot.
Some interruptions are a sign of dominance, or a way to control the direction of a conversation — but other times, people interrupt as a sign of intimacy or encouragement. You're more likely to interrupt your best friend than your boss, for instance, and some "interruptions" are more like interjections, facilitating the other person's point rather than hijacking their conversational space.
But in the context of a debate like this, interruptions are rarely pro-social. Usually the interrupter is seeking attention, or admonishing the interruptee, or both.
It's true that in Monday night's debate, Lester Holt interrupted Trump about twice as often as he interrupted Clinton. That probably had something to do with Trump's aggression and disregard for debate rules, as well as the need to do a bit of on-air fact-checking.
Statistically speaking, though, being a woman makes Clinton more likely to be on the receiving end of these kinds of interruptions — and, on top of that, more likely to be criticized for the way she responds to them.