Firstly I think it is relative to the education a person was able to obtain and how much they mentally evolve. I also reckon background plays a huge role but you will never quite know 'how much'.
But then I sometimes wonder. I heard this story of two educated young brothers that had an alcoholic father. The one brother drank like a fish and the other did not touch alcohol. When the first brother was asked why he drank so much, he replied, "well look at my father, what else could I do". When the second brother was asked the same question he replied, "well look at my father, what else could I do". So it gets down to a mentality aswell. Too many influences etc to pinpoint just 'how much'.