So my issue with this is that there are numerous counter examples that come readily to mind. I understand that the plural of anecdote is not data, and that naming random exceptions is generally a poor form of argument.

But, Barack Obama was in the "Choom Gang" when he was in high school, which roughly translates to the "Marijuana Gang". Think about that: The current leader of the free world smoked so much as a teenager that he and his friends named their group after the drug itself.

The last three US presidents have said that they smoked marijuana as teenagers and college students. Carl Sagan was a big proponent, and there are many other successful people in the worlds of politics, science and business who smoked pot casually at the least. If it does change the structure of the brain and impair long term memory, what exactly does that mean? Whatever the effect, it apparently doesn't disqualify people from being among the most intelligent and successful in the world.

Brain studies are useful, but we should be cautious when using them to draw sweeping conclusions about social policy.