So there appears to be this disease/disorder called Williams Syndrome. It's super rare but it biologically renders the person incapable of not trusting others. I'm not making this up.

http://www.scpr.org/news/2010/04/25/a-life-without-fear/

For the record I'd like to say that if/when I become a parent, this would be my worst nightmare because it would severely handicap my ability to talk to my own child since anyone who knows me is aware that half of what I say is sarcasm.