Turntable hum

If I hook up the grounding wire from my turntable to my amp the hum gets noticeably louder. What gives?

Sounds like an electrical issue as @almarg  suggested.
Btw, polarized plugs are not a “nanny state” encumbrance. They ensure that equipment which has a “grounding resistor” from neutral to chassis, usually several megohms, doesn’t inadvertently have the chassis become energized to hot lead potential by mistake. The whole concept is flawed of course.  ULc, CSA and UL allows this terrible method of false economy for a manufacturer to avoid cost of a proper grounded power cord. 
Run a wire from a screw on the bottom of your turntable and hook it up to the ground right along with the grounding wire.  Completely fixed the hum problem on my Pioneer PL 630.