‘He or she’ vs. ‘they’ in English

Traditionally, in English, when you had to use a pronoun for a person whose sex was unknown because it had been previously referred to as a “child”, “doctor”, “researcher”, and similar, you would say “he”, “him”, “his”, or “himself”, depending on the context. For example:

• Take care of your child. His life depends on you.
• You should see a doctor. He will know what to do.
• Every researcher has asked himself at some point of his career whether his contribution to science was good enough.

Although the pronouns were masculine, the child in the first example could have been a girl, and the doctor and the researcher could be women. However, this usage is now becoming outdated, as it is seen as stimulating gender inequality. There are several options to mitigate the issue. You can use “he or she” instead of “he”:

• Take care of your child. His or her life depends on you.
• You should see a doctor. He or she will know what to do.
• Every researcher has asked himself or herself at some point of his or her career whether his or her contribution to science was good enough.

As you can see especially in the third sentence, this grammatical tool soon becomes clumsy if you use it too often. Another possibility is to reword the whole sentence using the corresponding plural nouns, but this doesn’t always work:

• Take care of your children. Their lives depend on you.
• You should see doctors. They will know what to do.
• All researchers have asked themselves at some point of their career whether their contribution to science was good enough.

Finally, it is becoming increasingly widespread and regarded as completely grammatically correct to use a plural pronoun for a singular noun to refer to it in a gender-neutral way:

• Take care of your child. Their life depends on you.
• You should see a doctor. They will know what to do.
• Every researcher has asked themselves at some point of their career whether their contribution to science was good enough.

To me, personally, such sentences sound odd because they break the most elementary grammatical rules I have been taught in school. However, there seems to be an inevitable trend for this usage to replace gender-neutral “he, his, him” in English. Note that some authors went as far as to always use a feminine pronoun, for example

• Every researcher has asked herself at some point of her career whether her contribution to science was good enough.

Please, don’t do that. It is unnecessary and confusing, unless it is clear from the context that you mean only female researchers.