About one in one hundred people are probably psychopath in the general population. When you look at some sub-populations you find higher representations of psychopaths, or of tendencies associated with pyschopathy. One clear example given in the article is in the prison population. Other examples include high-level businessmen and politicians. Obviously saying "there's an over-representation of psychopaths among politicians" is not the same as a simplistic "all politicians are psychopaths".

Better quality research is needed, and pop-psychology books are pretty frustrating.