I was just reading through the wonderful debriefing that is Skepchick quickies, and came across this Scientific American piece about Sherlock Holmes and logical fallacy.The gist of it all is, that we are more likely to choose two things to describe something than one.  The article cites a scholarly article where people were described with various attributes and then choices were given for the subject to guess which phrase best described them.  When told the character was good at math, the subjects were far more likely to guess that they were an accountant and something else -- even when the choice 'accountant' by itself was available.

The logicallly fallacious part of this all, is that a subset (i.e. accountant and something) is smaller than the set it's a subset of (i.e. accountants).  Because of that, it's much less likely that the conjuction of the two sets (accountants and gymnasts, say) is the correct answer.

I may have to go back and read some Holmes to see if I can notice this quality of his.  I think I read most of it on a flight from Japan to Germany by way of China, and I'm pretty sure my brain blocked most of that to keep me sane.