On January 4, PJ Eby sent around an email linking an... interesting... website. The claim on the particular webpage he linked was as follows:

• the normal span of your breath is critical to how well your mental faculties can function
• the best activity for increasing your breath span is held-breath underwater swimming
• this also results in an increase in intelligence caused by a permanent increase in blood flow to the brain
• being fully underwater is important to the practice because it induces the diving reflex response

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.