The characters of Joel and Felicity have Asperger's syndrome. Joel is clearly diagnosed and a lot of his self-talk reflects this, there's real examples of concentration techniques, anxiety-breaking distractions and most of all, the questioning of social behaviours and body language.
Felicity’s Asperger’s seems to be undiagnosed. It's never stated in the book and it may be a figment of my imagination but I've noticed that a few other readers have made the same connection. It's pretty likely that she has Asperger's. Felicity traits are more typical of females with Asperger's which makes it less detectable and usually less limiting but it nevertheless comes into play at opportune (and sometimes inopportune) moments.