Link: https://slatestarcodex.com/2014/04/28/the-control-group-is-out-of-control/

I.

Allan Crossman calls parapsychology the control group for science.

That is, in let's say a drug testing experiment, you give some people the drug and they recover. That doesn't tell you much until you give some other people a placebo drug you know doesn't work – but which they themselves believe in – and see how many of them recover. That number tells you how many people will recover whether the drug works or not. Unless people on your real drug do significantly better than people on the placebo drug, you haven't found anything.

On the meta-level, you're studying some phenomenon and you get some positive findings. That doesn't tell you much until you take some other researchers who are studying a phenomenon you know doesn't exist – but which they themselves believe in – and see how many of them get positive findings. That number tells you how many studies will discover positive results whether the phenomenon is real or not. Unless studies of the real phenomenon do significantly better than studies of the placebo phenomenon, you haven't found anything.

Trying to set up placebo science would be a logistical nightmare. You'd have to find a phenomenon that definitely doesn't exist, somehow convince a whole community of scientists across the world that it does, and fund them to study it for a couple of decades without them figuring it out.

Luckily we have a natural experiment in terms of parapsychology – the study of psychic phenomena – which most reasonable people believe don't exist, but which a community of practicing scientists believes in and publishes papers on all the time.