But there is a testable claim here [...] The possible gains seem to me to outweigh the costs of the training and the low prior probability of the claim.
Most quack/traditional remedies are testable. Indiscriminately testing them in hopes of finding ones that work is potentially fruitful, but given the current level of scientific knowledge it's probably orders of magnitude more marginally useful to focus on testing the hypotheses located based on biological/medical expertise.
On January 4, PJ Eby sent around an email linking an... interesting... website. The claim on the particular webpage he linked was as follows:
This site is part of a sales pitch, so many of the claims are stated in hyperbolic language. I've already noted one factual error: the webpage claims that being underwater triggers the diving reflex, while in fact (or at least, according to Wikipedia) the diving reflex is triggered when one's face is immersed in water colder that 21 °C.
But there is a testable claim here: learn to hold your breath for longer periods of time -- particularly in conditions that elicit the diving reflex -- and you will see increased intelligence. I know that some readers of LW regularly train and test their intelligence, so I offer this as an easily implemented potential method. The possible gains seem to me to outweigh the costs of the training and the low prior probability of the claim.