I received an interesting email from the lead researcher on the work reported in the earlier post on prosopagnosia:
Dear Razib, I appreciate your comments (and scepticsm) about our reported work and the thoughts about continuums (aspergers/autism). One possibly clarifying point -- Face recognition is just one aspect of face specific processing. Humans are are also adept at judging emotion, mood, intention, age, attractiveness in faces. Its our experience that the majority of prosopagnosic individuals (with some notable exceptions) are pretty much normal in performing these arguably more important perceptual chores. Their problem is in recognizing faces, particularly faces out of context. This can occur frequently in modern urban society with its crowds, great mobility plus the surfeit of facial images. It probably occurred less often in traditional societies, thus reducing selection pressure. Best, Ken
In other words, the frequency of this problem should vary with demographic history of the population that the individual is drawn from.