[calljack]

They is a perfectly correct pronoun when you don’t know the sex of the individual. How can you construct a sentence with an unknown gender. You call them they.

[posterchild]

I just created such a sentence by using phrases or words such as ‘staff member’ and the plural form of ‘individual.’

[Verginius Rufus]

Our Modern English pronouns go back to Anglo-Saxon pronouns as they evolved through the Middle Ages. Our third person plural pronouns are actually of Scandinavian origin (they/their/them), but 'em may be a survival of an Anglo-Saxon pronoun rather than a shortened version of "them."

We could use a neutral third person singular pronoun (either he or she) but evidently people in the Middle Ages saw little need for such a pronoun--perhaps because gender roles were more clearly differentiated than today. "It" can be used in some situations but doesn't seem appropriate when referring to adult human beings.

So that's why people use "they" when it refers to one person but they don't know whether it is a man or a woman, or want to leave it undetermined. It does come across as grammatically incorrect to people who care about correct grammar, but they are probably a dying breed.

[HIDEK6]

You are a philistine.

“They” is plural.