Er, why don't you understand, given that you've deduced it's something that some people have from birth?

BTW, I haven't read the book, but I wonder if part of the argument that it inhibits language acquisition is to do with the subtleties of intonation - where the pitch is contextual rather than absolute. To interpret the same intonation in different people, you'd need to slide up and down the scale depending on who was talking. Which is interesting to consider when you link it with the fact that a common feature of autism (strongly associated with a weakness in interpreting the emotional states of other people) is a monotonous voice with very little subtlety of intonation. I'd better go read the book now, hadn't I?

I would have thought that most people have a good sense of frequencies - in that they can tell if a singer or musician is 'off'. The ability to recognise a note and give that note its correct nomenclature is a learned skill and therefore should not be part of a discussion about hearing. I wouldn't know a B flat from an A sharp, by name, unless the difference was demonstrated to me. If a musician (particularly those who play stringed instruments) cannot 'tune' their much beloved device then why (and how) are they playing?
[Rosie] On reviewing my post I think I sort of agree with you.
[Projoy] I'm not sure. Some oriental languages seem to rely on intonation changes for the meaning of various words and phrases - though English does the same to some extent - but I suspect that your comment regarding a 'sliding' scale must be true and would certainly not rely on perfect/absolute pitch, just the variation in such.

[Duj] Pianists can't always tune their instrument, and they seem to get on OK. :)

(Projoy) What I can't understand is why everybody hasn't got it from birth. As I said, it's only a form of memory, like remembering colours. (Dujon) Most people can tell if someone is out of tune, but only relative to some other pitch which is assumed correct. Most people who play a guitar can tune it so that the instrument as a whole is in tune. I have a friend who plays a few tunes (only at home), and plays rather well, and his guitar is in tune with itself but at least three semitones flat. He has a good sense of tuning but none whatever of absolute pitch. I tuned it up for him but he said the strings were far too tight so back down it went.