I have a couple of extremely bright friends (one of them was possibly the smartest guy I've met) who became pot abusers for all manner of reasons. One wanted (and had the skills) to be a stock broker. He spent the first summer of university trading and made a really strong start. But from the second year onwards his pot habit inhibited his learning/development and he now does door-to-door collections for a charity on minimum wage. He was probably my second closest friend at university so I am absolutely sure that cannabis was the cause of his decline.