Took me a few minutes. I kept trying to bring in extra information as if it was a joke. Once I considered that ALL the information needed was included in the question I got it right away.
Seriously though, just because I might be able to think like a psycopath when I want to, it doesn't mean I lack a conscience or that I think like that all the time.
This was a test by a famous American Psychologist used to test if one has the same mentality as a killer.
Who was he and does he have any more? I like these kind of riddles that don't depend on puns and stupid double meanings.
Also if he CAME UP with the test, doesn't that mean he himself has the metality of a killer?
This is, of course, not a real method of detecting psychopaths: http://www.snopes.com/inboxer/hoaxes/sister.asp
But it's a fun riddle anyway!