HD is not always hereditary and can be caused by strenuous activity at a young age or injury. It makes sense to me that if early spay/neuter can affect the way a dog structurally matures, it could be linked to(I never said cause) HD. I'm not saying any study is gospel, only that they're worth looking at IMO.
AGREED!! There are loads of cases of HD in dogs with awesome, tested backgrounds who develop HD anyways. It has been shown it can be environmental too.
I recommend waiting until a female is about eight months old to spay. Hopefully this will miss her first heat (usually at about nine months of age) but allow her to grow and fill out for some time). Males I suggest be neutered at around 14 months of age so he can bulk up and develop muscle before being done.