If something is a learned behaviour, it necessarily requires a teacher, JJ.  I appreciate that that teaching may not be conscious or active, but passive teaching is just as bad.  For instance, if I as a parent, fail to criticise racist behaviour or cruelty to animals in front of my children, I am - effectively - condoning this kind of action, thus teaching them that its OK.

No it doesn't 'necessarily require a teacher'.  You can learn simply by observing the state of the world, by doing things and observing their effect, by making mistakes etc.  But so-called learned behaviour isn't really the case in point here, it's the effect of the world and its imperfections that causes us to become what we are.  We don't learn so much as become.