"Their" used as a pronoun in order to avoid the sexist use of "his" and the clumsy use of "his/her"

Gender-neutral language movement

In the late 20th century, the feminist movement expressed concern regarding the use of generic he in the English language. The feminist claim was that such usage contributes to an assumption that maleness is "standard," and that femaleness is "different". They also claimed that such use is misogynistic. One response to this was an increase in the use of genericshe in academic journal articles from around this time. However, the more common response has been prescriptive, with many institutions publishing gender neutral style guides, notably in government, academia and publishing.^[35] For example, The Cambridge Guide to English Usage (2004) expresses several preferences. "Generic/universal their provides a gender-free pronoun, avoiding the exclusive his and the clumsy his/her."

It avoids gratuitous sexism and gives the statement broadest reference....They, them, their are now freely used in agreement with singular indefinite pronouns and determiners, those with universal implications such as any(one), every(one), no(one), as well as each and some(one), whose reference is often more individual....For those listening or reading, it has become unremarkable—an element of common usage.^[