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)?