Why is it that the people who insist that gender is an exclusively biological product never have any proof?

They insist over and over that science has "proven" this when there is no credible evidence for it?

Here is my challenge:
Can anyone find credible evidence from an authoritative source stating that gender is soley (or overwhelmingly) the product of biology?

Why would social scientists differenciate between SEX and GENDER if the two are interchangeable?

The both of you are wrong. Gender isn’t exclusively biological, but it isn’t exclusively social either..

Gender is both biological and social.