I'm revising a text containing these two sentences:
a) Students’ sex was coded by using their first names.
b) Students’ birthday was self-reported in the survey.
I'm sure the correct version should have "students' birthdays" because the rule in English is to use the plural for the possessed objects, even if the possessors only have one each.
But if I apply this logic to sentence a) about sex, I get "students' sexes" ... which seems ridiculous. Why? They are both potentially plural, yet each student has only one sex, and one birthday.
I wonder if the parameter "sex" has only a superficial resemblance to the parameter "birthday". Have you any ideas? Many thanks.