http://www.pnas.org/content/110/11/4251.abstract
It seems the original authors didn't even bother to look for confounding factors, such as socioeconomic status or use of other drugs.
Hmm, let's dig further. The home page of the TPRC lists a single funder: the NDIA.
https://childandfamilypolicy.duke.edu/project/transdisciplin...
That would be the National Institute on Drug Abuse, an institution which has a pre-established position on marijuana and other drugs, and stands to gain financially the more problems that are perceived with recreational drug use.
The following government piece on drugabuse.org does not mention the serious criticisms, despite heavily quoting from the journal in which they were published 6 months prior. Since it streches credulity to think they didn't know about the problems raised, the logical conclusion is that the authors felt their piece would be stronger without mentioning them (versus offering any counter-rebuttal).
https://www.drugabuse.gov/news-events/nida-notes/2013/08/ear...