The Red Flags
Before you skip down to the comments to submit your passionate defense of Holmes' mental state, we're not the only ones who think he shows up on the autism spectrum. Holmes' hyper-keen observational skills, social mannerisms and overall personality have fueled Asperger's rumors everywhere from Holmes fan forums to Asperger's support forums.
"Holmes, stop looking through my stuff for clues. This is the reason no one else will lodge with you."
The first thing to keep in mind is that the character isn't just portrayed as being really smart -- he is obsessed with certain subjects and totally excludes all others. In one of the Holmes stories, A Study in Scarlet, he doesn't know that the Earth revolves around the sun (because, he says, the information doesn't have any effect on his everyday life). These uneven obsessions with random topics -- in Holmes' case, things like tobacco ashes and regional soil consistency -- are not signs of an enthusiast; they are symptoms of a disorder. Or, as the Yale Child Study Center puts it, Asperger's sufferers show "...a narrow range of capacities for memorizing lists or trivial information, calendar calculation, visual-spatial skills such as drawing, or musical skills involving a perfect pitch or playing a piece of music after hearing it only once."
"Care for a 70th rendition of 'Baa Baa Black Sheep,' my dear Watson?"