Historically, in British English at least, if you didn’t know someone’s preferred gender it was considered grammatically correct to use he to refer to them, even if they might be female. Based on the justification that ‘the masculine includes the feminine’, this means that all of the following would be considered fine examples of English usage:

• The driver in front is swerving like he is drunk.
• A scientist is a fountain of knowledge; he should be respected.
• Any student wishing to answer a question should raise his hand.
• Everyone should consider his own family when choosing how to vote.

When you picture the people referred to in these scenarios, were any of them women? Or, to put it another way, were any of them any identity other than ‘male’? Evidence from psychological experiments has shown that the pronoun he (in all its forms) evokes a male image in the mind. Its use as a ‘generic’ pronoun, in contrast to what grammarians of old seemed to think, actually makes it harder to read and process sentences with stereotypically feminine referents (i.e. A childminder must wash his hands before feeding the children.).

So if you don’t want to go around assuming that all the world is male by default, what do you do? Luckily, there is a solution to this problem: if you don’t know a person’s gender identity, you can use the pronoun they to refer to them. There may be a mental screech of brakes here for those of you who were taught that they is a plural pronoun, but actually, it’s more versatile than that. Try using they for he in all of the sentences above. When thinking about the scientist or the driver, was there suddenly more than one? No. Indeed, singular they has been shown not to interfere with mental processing in the way that generic he does.  I used it in the first sentence of this post and I’ll bet you didn’t even notice it. (Go on. Check.)

For those of you still not convinced, the use of singular they is widespread in spoken and written English. It’s highly likely that you use the form yourself without even thinking about it. In British Pronoun Use, Prescription and Processing (Palgrave 2014) an analysis of this type of pronouns demonstrates that singular they is ubiquitous in British English. If you still need more convincing, here’s a link to an extremely favourable review of that study just published in Language and Society.