A lot of us understand biological sex with a pretty fateful underpinning: if you’re born with XX chromosomes, you’re female; if you’re born with XY chromosomes, you’re male. But it turns out, our relationship to the opposite sex is more complicated than we think.

This episode was reported by Molly Webster, and produced by Matt Kielty. With scoring, original composition and mixing by Matt Kielty and Alex Overington. Additional production by Rachael Cusick, and editing by Pat Walters. The “Ballad of Daniel Webster” and “Gonads” was written, performed and produced by Majel Connery and Alex Overington.

Thank you to the musicians who gave us permission to use their work in this episode—composer Erik Friedlander, for "Frail as a Breeze, Part II," and musician Sam Prekop, whose work "A Geometric," from his album The Republic, is out on Thrill Jockey.

Radiolab is supported in part by Science Sandbox, a Simons Foundation initiative dedicated to engaging everyone with the process of science. And the Alfred P. Sloan Foundation, enhancing public understanding of science and technology in the modern world. More information about Sloan at www.sloan.org.