The best way to test this would be to approach it like Intelligence research originally approached that hypothesis for IQ scores: by testing a reasonably large set of twins separated at birth. That's such a tricky criteria to come by, I can't imagine it'll be easy to do again, but it did help cement the idea that IQ was likely not significantly impacted cultural/environmental factors, and thus possibly a result of subtle genetic traits. The interesting thing about those studies though, was that the separated twins tended to have a lot of other unexpectedly subtle quirks in common too (such as sense of humor). So in that context, I actually wouldn't be surprised if rationality falls under those similarities as well.
