I, for one, would never have predicted this:

“[I]ndividuals who started using cannabis in adolescence and used it for years thereafter showed an average eight-point IQ decline.

“Quitting or reducing cannabis use did not appear to fully restore intellectual functioning among adolescent-onset former persistent cannabis users,” [Professor Terrie Moffitt] said.

Although eight points did not sound much, it was not trivial, she warned.

It meant that an average person dropped far down the intelligence rankings, so that instead of 50 per cent of the population being more intelligent than them, 71 per cent were.

Meier, et al, PNAS, Aug 27, 2012.