"Zodiac...does that mean bigotry IS psychosis?" Technically, no.
"Or only if it is 'extreme bigotry'?" The study apparently only dealt with that.
"Is 'non-extreme bigotry' not, then, considered psychotic?" Certainly not, but the fact that extreme bigotry's slightly correlated with psychosis would make a good attack on bigotry in general. That is, if you could count on bigots to get it.
"Could you please define 'normal bigotry' as opposed to 'extreme bigotry'?" No. I'm just quoting the article. I did find this other article - here:
http://tinyurl.com/abqty - that explains more about the same debate, if not the same study.
"Is the implication that only inmates are capable if 'extreme' bigotry? Or only that they should be given anti-psychotic drugs for it?" No. The implication is probably that the easiest test group was inmates. The other implication is that defining bigotry as a psychosis would make it a legally admissable defense for hate crimes, and that, as you've pointed out, the vagueness of the term "extreme bigotry" means almost anyone qualifies.
Surely the study was done in some way that accounts for the placebo effect, even if by just admitting that the correlation hasn't been tested for the placebo effect yet. Given that the inmates wouldn't have automatically expected that anti-psychosis drugs would reduce their bigotry, you can hardly expect the placebo effect to be strong in this case.