FWIW, at least according to Hillel Wayne's summaries of the state of our knowledge on this stuff (www.hillelwayne.com), all three of those have been studied, an

FWIW, at least according to Hillel Wayne's summaries of the state of our knowledge on this stuff (www.hillelwayne.com), all three of those have been studied, and the only one that's been shown to be clearly beneficial is unit tests. (Not TDD, mind. It apparently doesn't matter much whether you write tests first or second, just that you write them at all.)