When I play mass effect, and the rest of the series, I have noticed the Shephard has some signs of asphergers, at least in my opinion.
I noticed that Shephard tend to talk in a monotone voice. Even Mordun Solus and EDI have more personality in their voice.
I also noticed that shepherd doesn't seem to know much about the mass effect universe. Even if you ignore the question choices in the speech circle, Shephard tends to know more about the military then anything else.
Another thing I noticed that Shephard tends to be truthful to a fault. He almost never lies. Even when he does, it is a half truth. And even at the worst, he is horrible at lying.
Granted I am not the best it comes determining a person's personality. Also, my asphergers might be making my case too personal, which is what video games tend to do.