Greetings and other salutations are a very interesting area of linguistic behaviour, and one I intend to pay more attention to later in my career. They are also quite relevant to my current research since they seem to be the focus of a great deal of variation.
Alan Cruttenden notes tantalisingly, in his book Intonation:
There are certain areas which are particularly susceptible to idiosyncratic uses of tones: Greetings, farewells and social formulas are one such area: the conventional way of intoning the equivalent of Good morning will vary from language to language; moreover variation within one language in such areas will be sensitive to very subtle social conventions. (1986:169)
This is so tantalising because he doesn’t really say much else about it. If anyone is aware of anyone who says much more, particularly with good empirical data, please let me know.
In any case, it shouldn’t surprise us that greetings vary like this: they typically occur early in discourse, and are more or less obligatory in many social contexts, yet carry very little propositional information. They are therefore a very good place to use linguistic form to mark one’s identity and to mark oneself as belonging to one group or another. With this in mind, it’s notable that they usually require a specific response, which is not always obvious. At least for a while, the expected response to ‘How do you do?’ was ‘How do you do?’ (it probably still is for most people who employ the greeting). Similarly, when I started secondary school, I replied to ‘all right?’ as if it were intended as a question, until someone pointed out to me that the expected response was ‘all right?’
I still get caught out by this kind of thing. When I pass people I know in the street or corridor, I tend to say something like: ‘Hiya!’ Frequently they reply (or even get in first) with something like: ‘Hi, how are you doing?’
Only very rarely do they expect me to tell them (and when they do, this is signalled by their stopping). Ignoring the question, however, feels impolite. It’s not, of course, and one day (I hope) I’ll actually get used to this.