For some things, a real scientific experiment isn't possible, because there's isn't enough control. Equities is one of those things. For equities, when you see a performance that beats the market averages, it wise to assume it's because of chance, not genius.

Also, there are any number of convincing scams meant to distort one's thinking. My favorite is one I call "Miracle Man", in which a manager mails you six perfectly correct monthly predictions, all in advance of the real market they predict, then asks to take over your portfolio. Hard to believe, but it's a scam, it isn't real:

http://arachnoid.com/equities_myths/index.html#Miracle_Man