I don't think that's a very safe assumption at all. Lots of athletes do strange things for no good reason. See: Patrick Roy stepping over the blue lines compulsively throughout his entire career. I wonder if he still does it now that he's a coach.
I know I play differently if I think something's different with my equipment, even if I have little evidence to support that.
Those examples have nothing to do with wanting to, consciously or inadvertently, influence the outcome of the experiment. Double-blind only helps in that situation.
Nobody cares if the players want to influence the experiment. The point is to determine whether camouflage pads are more effective than regular pads, which isn't possible. There are simply way too many variables that you can't control.
As has been pointed out by others, with enough data, we can infer causality with reasonable confidence. It's not as good as being able to design the experiments ourselves, but it's all we have.
The alternative is to throw up our hands and declare that everything outside the lab is outside of science.
And as I keep pointing out, you'll never have the quantities of data required to make that kind of statement with any confidence.
For instance, why do the Leafs get halfway through the season and then start sucking? It's been the same story 15 years running now. You'd like to be able to point to one factor that was constant, but every element of the team and the league has changed at every level, from the coaching to the management to the players to the rules to the rink they play in.
The only thing that hasn't changed is the size of the puck, but nobody's claiming that that's why the Leafs can't win.
Moronic tangent over! I shouldn't post after midnight.
I know I play differently if I think something's different with my equipment, even if I have little evidence to support that.