I completely agree, this is a consequence of the patriarchy pretty much by definition. The smoking gun is that girls become more talkative specifically when men

I completely agree, this is a consequence of the patriarchy pretty much by definition. The smoking gun is that girls become more talkative specifically when men are absent, so girls aren't just less talkative in general, it's specifically a reaction to the presence of boys. By "exact mechanism" I meant more specifically why the phenomenon happens. Are women encouraged to be quiet when men are talking, or are men encouraged to interrupt women, or both? In a truly egalitarian society, would men interrupt less or would women interrupt more?