Use of they as singular [duplicate]

Possible Duplicate:
Is it correct to use “their” instead of “his or her”?

I, and many people I have heard, occasionally use 'they' and 'their' in the singular when gender is not known. For example,

Someone burgled our house yesterday. They must've cut their head on the broken window, because there is blood there.

I would like to know:

  • Is this incorrect or not used in some places?
  • If not, I have read, such as in style books, the awkwardness of not assuming certain people are a 'he' or 'she' and using 'his' and 'hers' with them respectively, thereby having to say 'the reader... he/she should...his/her'. Why don't they simply use 'they' and 'their'?
  • Is this a recent development in English?
  • Is this type of usage present in other languages (pronouns for both genders)?

This is called singular they and is highly controversial. Some forms are more acceptable than others. It is a old usage and Caxton wrote in the 15th century something like

Each of them should ... make themself ready

though I suspect many people might now prefer themselves to make the clash of grammatical number less obvious. Here are are four examples of which the first is probably most widely acceptable, and the fourth least.

  • Students should work by themselves
  • Each student should work by themselves
  • Students should work by themself
  • Each student should work by themself

The answer where possible is to rewrite to either of

  • Students should work alone
  • Each student should work alone

and then there will be no objection.