Immunity to psychology is a bit of a misnomer, that is, it doesn't really describe itself properly. Under the character's section of the rulebook it appears that any character joining a unit immune to psychology does themself gain that immunity. Fair enough.
However, I had always understood stupidity and frenzy to be psychological traits but these traits are not in fact impacted by immunity to psychology.
Immunity to psychology only affects terror, fear and panic.
A character or unit immune to psychology still has to test for stupidity and frenzy.
When you see the expression immunity to psychology replace it in your mind with immunity to "some psychology" or immunity to terror, fear and panic.
Confused ? I was but I have reread the relevant sections several times now and I'm fairly certain I have made the correct interpretation.
If you disagree or can clarify further please feel free to comment.
