Classroom participation – is there a gender issue?

One part of the difference is not that women are different in their willingness to ask questions, but anyone visibly different is. I was an undergrad in a class of 44. There were 4 women: two of one visible ethnicity, and two of another. My hair was a different colour and length than the other woman of my ethnicity. This meant everyone knew who we were. If we were late, asked a question that showed we hadn't been listening, or did anything else, everyone knew who did that. In contrast there were 5 or 6 white guys with short dark hair and football player builds who wore leather class jackets and sat at the back. There were 5 or 6 guys of several other ethnicities, and 5 or 6 nerdy glasses-wearing white guys with bad fashion sense. I got some of them confused sometimes. They all had somewhere to hide. They could blurt something out without much consequence.

If you would like more people to participate in your class, make sure you don't punish "bad" participation at all. It may be fun to be the person who answers a question with "I see somebody skipped the pre class reading" and then just turn away, but it ensures a large chunk of the class will not ask that sort of question again, and those who do will use up class time with defensive speeches about how they were reading the blah and blah "and on page (flip flip yes here it is, 73) it says (read sentence) and so I was wondering whether "permanent" is really appropriate here" and so on.

You can also call on students who don't ask many questions, asking them questions. My PhD supervisor did this constantly. Like 5-10 times an hour. "... and so if we reduce this completely, the result is ... Mrs Gregory?" (he was such a formal guy, called us all Mr and Mrs) and if I was following I would say the next line in the work, and if I wasn't I would say "I don't know" and he would call on someone else. This kind of approach keeps people engaged (you never know when you'll be called on) and gives everyone practice speaking up in class. (Plus it teaches you when to say "I don't know" - and he never had a problem with us not knowing.)

The point is that in an ideal world we would like people's academic and later career success to be determined solely by their intrinsic talent and by how hard they work. Unfortunately other factors that ought to be irrelevant get in the way and also end up having an effect. One of many examples is the particular issue you mentioned, which is the fact that someone who is very shy and never asks questions is likely to hurt their success in various ways.

Now, this is true irrespective of gender, but it becomes a gender issue if/when/to the extent that (in a particular setting or context) female students may find it more difficult or intimidating to ask questions than male students, with the effect that they end up (statistically, as a group - obviously this does not necessarily apply to any individual student) unfairly disadvantaged in similar ways to how shy students in general are disadvantaged. I can't say from personal experience whether that's true, but I've certainly read many testimonials (that seemed credible to me) to that effect: a female in a classroom or workplace that is almost exclusively male (and often hostile to women in subtle ways that are hard to gauge or quantify) may find it more difficult to make her voice heard effectively than her male colleagues do. I don't know about you, but it sure sounds like an "issue" to me.

Hope this clarifies things. And please listen to your adviser's advice to speak up more in class. That sounds like good advice regardless of whether you are male or female.

Is this a real phenomenon?

Yes. Gender differences in classroom participation have been noted and explored by sociologists probably since the moment women were admitted into college classrooms with men. Five seconds with google gave me this article whose introduction gives an overview of the research into this question going back to at least the '70s.

Why is it an issue?

Because classroom participation is associated with higher grades/achievements. I'm not in the field so I can't tell you about the state of the evidence for causality here, but there's several reasons to think that there is at least a partial causal relationship (such as what you said about recommendation letters), and that if women were able to engage in the classroom in the same way as men, this would improve their grades. (Note that this isn't quite the same thing as just talking more: there are some complicating factors, such as women being perceived as talking too much/dominating the conversation as soon as 30% of speaking time is used by women. If women 'just speak up more' they get different results than when men do.)