Usage and Grammar

Q. CMOS Editors, which way does Chicago lean—singular or plural verb in “One in ten people is/are affected”?

A. In formal written English, expressions like “one in ten” take a singular verb (is), since one is a singular noun. In informal speech and writing, the plural often sounds more natural.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]