

Scott Alexander has told many stories about why he is pseudonymous. In 2013, when he was applying for medical residency, he told a simple story which begins with an interviewer saying he would have to take his LiveJournal down if he were accepted:
I originally deliberately linked this blog and some of my other writings to my real name in order to bury Google evidence of certain stuff I did online when I was a dumb teenager, but that mission is pretty thorougly (sic) accomplished and now I agree with my interviewer that even if I don’t get hired (cue serious of elaborate ritual hand gestures to ward off unthinkable misfortune) it is to my benefit to switch back to my normal totally un-Google-able pseudonym of Scott Alexander (well, actually just my first and middle name).
FYI, this is why its a red flag that someone has public profiles on dozens of sites. Its a standard technique to bury negative posts in Google results, PR firms will write the profiles and get you a few press releases and talks to help. I hope this is the last thing I have to say about this particular creep.

if you held a gun to my head I would guess it would be his racism (he thinks that different groups have natural places in society driven by their inborn hereditary differences), but I don’t see any point in trying to track it down. He is clever but uses his brains for very common, very bad ideas and the things he has done since 2013 are much worse than the things he could have posted as a young man.