The GSS says whites wanting to prohibit marriage between blacks and whites are dumb, but not crazy: The stereotype of a white nationalist (WN) is a inbred hillbilly who's missing some teeth and who missed school after about the 6th grade. But the folks of this persuasion on the Internet I've talked to are generally a smart bunch. My students stare at me in disbelief when I tell them that William Pierce had a PhD in physics. I imagine people who debate on the Internet are the cream of the crop, but I don't know--the dumb stereotype may be the product of hostility. And the image is not just one of low intelligence, but also mental illness. Just yesterday, I talked to a guy who lived in Hayden Lake, Idaho for many years and was on friendly terms with a number of WNs, but he told me that when he moved from there, the WN types were getting pretty psycho.
I couldn't find a perfect question to identify WNs, but the General Social Survey did ask if you are in favor of a law forbidding marriage between blacks and whites. Respondents were also asked if they have ever had a mental illness. Here are the results:
Against laws prohibiting intermarriage 102.1
For laws prohibiting intermarriage 92.2
Percent having had a mental illness--whites
Against laws prohibiting intermarriage 8.3
For laws prohibiting intermarriage 1.7
Keep in mind that that sample of those in favor of this type of law is small--only 59 people. But going on what we have here, these folks have a low average IQ but are less likely to have experienced some kind of mental health problem.