There's an urban legend that this riddle is easier for psychopaths:
A woman meets the man of her dreams at her own mother's funeral, but doesn't get his number. A few days later, the woman kills her own sister. Why?
If you find the riddle difficult, it's because your social mode of analysis is jumping in inappropriately. I could easily imagine that someone who was deficient in that kind of analysis might work it out faster.
Damn. I fail. And it is damn obvious too!
The Wason Selection Task is the somewhat famous experimental problem that requires attempting to falsify a hypothesis in order to get the correct answer. From the wikipedia article:
Aside from an illustration of the rampancy of confirmation bias (only 10-20% of people get it right), the task is interesting for another reason: when framed in terms of social interactions, people's performance dramatically improves:
However, apparently psychopaths perform nearly as badly on the "social contract" versions of this experiment as they do on the normal one. From the Economist:
The original (gated) research appears to be here.