Here are several plots to consider:
As this plot shows women earn more PhDs than men. (And if it isn't clear, click on the above link and read the report in detail.) Furthermore, this trend seems to hold across most fields.... except the physical sciences/math/CS/engineering fields which does not at all surprise me.
For the curious, the above plot shows how enrollment breaks down by ethnicity and gender. For some reason the only group to be predominately men is "temporary residents".
The above plot here shows the percentages of doctoral degrees awarded by field. It looks like engineering and education takes the cake.
Now I will move on to acceptance rates. For those interested, here are how acceptance rates break down across fields and degree types:
And lastly, the below plot shows how the popularity of fields have been changing recently. It seems like a lot more people want to go into health sciences and public administration than just a year ago. (Ouch math and computer science!)