Original article published to The Washington Post Jan. 18, 2016 by Christopher Ingraham You might have heard that smoking marijuana makes you stupid. If you grew up in the ’80s or ’90s, that was more or less the take-home message of countless anti-drug PSAs . In more recent years, it’s a message we’ve heard — albeit in more nuanced form — from Republican candidates on the campaign trail and from marijuana opponents at the state-level . The contemporary version of argument can be traced to a 2012 Duke University study , which found that persistent, heavy marijuana use through adolescence and young adulthood was associated with declines in IQ. Other researchers have since criticized that study’s methods . A follow-up study in the same journal found that the original research failed to account for a number of confounding factors that could also affect cognitive development, such as cigarette and alcohol use, mental illn...