There's a new study out of Dartmouth Medical School linking exposure to R-rated movies and early drinking.
The obvious reason, which is pointed out in the linked Business Week story, is that R-rated movies tend to show quite a bit of alcohol consumption.
The article also mentions that middle school children who are allowed to watch R-rated movies are more prone to "sensation seeking" and "risk taking," which I personally think is probably more on point.
But, of course, drinking would be included in the sensation seeking/risk taking categories.
The bigger picture is that R-rated movies are created for a mature audience; when immature children are exposed to mature material, they are bound to pick up some bad habits because they can't properly process what they are seeing.
They see people on the big screen excessively smoking and drinking and participating in other promiscuous behaviors, and they think that it's normal for adults to act that way, and so they try to emulate it.
It's not hard logic to follow.
However, instead of blaming movies, this should serve as a wake-up call to parents.
Pay attention to what your kids are watching.
Here's another good summary of the research from Lab Spaces.