Pseudonyms

I think Alice and Jonathan have pretty much nailed it. The reasons I can see someone choosing a pseudonym are:

1. To test the waters as a writer (new/different genre)

2. To better stand out or fit into their genre than their real name allows

3. To avoid or aim to be seen as a female or male writer (or neither)

4. To avoid being identified for whatever reason (personal, security, etc.)

5. To become the "character" of their pseudonym when writing (dressing-up for adults)

Interestingly, Robert Galbraith apparently had very average sales until the author's real name was leaked. Not a reason to use a pseudonym, then :)

If you write 'conventional' novels plus erotica then you might want to use different names for those two.

I've often wondered the same thing (idly, as I haven't concretely pinned down ONE genre, let alone two!). J.K Rowling used a pseudonym when she wrote her first crime novel - something she didn't do for 'The Casual Vacancy' - presumably to allow it to be judged for its own merits, and be taken as a piece firmly within the crime genre rather than as an author's 'dabbling'.

I suppose that another reason authors might choose to use a pseudonym would be if the two genres they were interested in had very different reputations. Rightly or wrongly, there is undeniably a 'hierarchy' of genres - I think that some people use their real names for the genre they'd most like to be associated with, and a pseudonym for the other. I don't know how advisable this would be from a marketing prospective, as I have no experience whatsoever!