Apparently their efficacy is greatly overrated, and it is a conspiracy amongst the establishment to try and make every paranoid person wear one.
Wearing a tinfoil helmet also reduces leg spasms caused by misderected mind control beams by 75%, too, so it is obvious that actually it is due to superior mind control resistance and mind independence techniques that this placebo effect occurs. More recent studies by the same authors have shown that there is no correlation between independent free thinking and the wearing of tinfoil hats.
Mind you, I still wear one due to pressure from my family.