Kelly- I don't mean to be a jerk, but you make some statements that are inaccurate and misleading in this article. You should really provide links for the articles you cite (http://www.pnas.org/content/109/40/E2657…
), to encourage cross checking of your statements.
For example, you state the following about the Duke paper: "Studies were performed periodically throughout their lives, and were carefully controlled to ensure test subjects did not have any other drug or alcohol addictions or psychological disorders, and were not high at the time of the test. They found that those who used pot more than once weekly before age 18 displayed more severely impaired intelligence, slower reaction times, shorter attention spans, and poorer listening skills than those who began using marijuana after age 18."
Before saying more, a definition of clinical dependence from the DSM-V manual where the researchers defined their qualifiers - "There is a pattern of repeated self-administration that can result in tolerance, withdrawal, and compulsive drug-taking behavior." Read more here or anywhere else (http://www.4shared.com/office/Qm28z-8H/D…
) but the bar is pretty high for this diagnosis (aka TOTAL POTHEADS *who have resulting problems*)
Your statement is inaccurate/untrue for a few reasons.
1. They demonstrated controls for other addictions, and that presence of other addictions did not affect their overall conclusion. BUT, they DID NOT exclude these individuals from their study as you suggest.
2. They show that people who used pot weekly before 18, and are continuing to use on a frequent basis at the age of 38 (Also, 2+ clinical diagnoses of dependence over the years of the study), demonstrate reduced IQ that persists even after stopping use. (Aside: this effect, btw, is ~7-8 IQ points, the error of an IQ test is likely 3.5-4, although they do not include this info in their study).
3. When considering ALL participants, which includes those clinically diagnosed as dependent 1, 2, & 3 times, they see reduction of "impaired intelligence, slower reaction times, shorter attention spans, and poorer listening skills." The individuals diagnosed as "clinically dependent" is where the majority of the statistical strength of their conclusions comes from. This is NOT, by ANY MEASURE AT ALL, the same as "...those who used pot more than once weekly before age 18".
Sorry for the long note, but I thought someone should say that this study isn't concluding or showing what the media has been saying. Is marijuana use at a clinically dependent level bad for you in these ways? Probably yes (almost definitely). Is it even worse when use starts before the age of 18, probably yes. BUT, I'm pretty sure this is not what you have stated here, and a correction should be issued if I read this study correctly.
But, the statement of "weekly" before the age of 18? That's almost definitely wrong based off of my reading...