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'Antisocial personality disorder' is what we used to call psychopathy. Psychologists are reluctant to label patients 'psychopaths' these days - for the obvious reasons.

Psychopaths make up around 1 in 150 of the general population - along with sociopaths this is thought to come to about 1 in 100.

Like you say, it's relatively common - of this forums 17,445 members - with all things being equal (assuming psychopaths are likely to participate in a public forum at the same rate as the general public) - there are likely 104 psychopaths.

Here are a few definitions, it is quite fascinating

http://www.cassiopaea.com/cassiopaea/psychopath_2.htm

http://www.crisiscounseling.com/articles/psychopath.htm

http://www.wisegeek.com/what-is-a-sociopath.htm

http://www.youmeworks.com/sociopaths.html