"Long before it's in the papers"
January 16, 2013

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Linkage between pot, low IQ “premature,” study says

Jan. 16, 2013
Courtesy of PNAS
and World Science staff

A study pub­lished last year that found a link be­tween smok­ing ma­ri­jua­na and IQ de­cline may have been based on in­com­plete in­forma­t­ion, new re­search sug­gests.

“The true ef­fect could be ze­ro” of pot on IQ, wrote Ole Ro­ge­berg of the Rag­nar Frisch Cen­tre for Eco­nom­ic Re­search, Os­lo, re­port­ing the new find­ings in this week’s early on­line is­sue of the jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

Al­ter­na­tive­, he added, there’s a chance pot might make you dumber, but in­di­rect­ly—not through phys­i­cal dam­age to brain cells, but through cul­tur­al fac­tors.

Ro­ge­berg said his com­put­er sim­ula­t­ions showed that a dif­fer­ent fac­tor af­fect­ing IQ, so­ci­o­ec­on­om­ic sta­tus, could have skewed the pre­vi­ous re­sults. The sim­ula­t­ions in­di­cat­ed so­ci­o­ec­on­om­ic sta­tus was enough to re­pro­duce the as­socia­t­ion be­tween IQ change and can­na­bis ex­po­sure re­ported in the pre­vi­ous study of a large sam­ple of peo­ple from Dun­e­din, New Zea­land, Ro­ge­berg said.

Pre­vi­ous pub­lica­t­ions based on the same Dun­e­din group in­di­cat­ed that early-on­set pot smok­ing was more com­mon in peo­ple with cer­tain risk fac­tors al­so tied to low so­ci­o­ec­on­om­ic sta­tus, he not­ed. Past re­search, he added, al­so sug­gests that chil­dren with si­m­i­lar IQs but dif­fer­ing so­ci­o­ec­on­om­ic sta­tus wind up, for what­ev­er rea­son, in, en­vi­ron­ments with dif­fer­ent cog­ni­tive de­mands. That in turn causes their IQs to branch apart.

“It would be too strong to say that the [pre­vi­ous] re­sults have been dis­cred­ited, but fair to say that the meth­od­ol­o­gy is flawed and the caus­al in­fer­ence drawn from the re­sults pre­ma­ture,” he wrote.

There is yet an­oth­er in­ter­preta­t­ion of the Dun­e­din re­sults, he added. Rath­er than pot dam­ag­ing brain cells, it could be that “heavy, per­sist­ent, adolescent- on­set can­na­bis use in­volves a cul­ture and norms that raise the risk of drop­ping out of school, get­ting en­tan­gled with crime, and oth­er such be­hav­iors.”

Fur­ther anal­y­ses of the Dun­e­din co­hort could help dis­tin­guish be­tween com­pet­ing in­ter­preta­t­ions, he went on, and pol­i­cy im­plica­t­ions might dif­fer de­pend­ing on wheth­er there is an IQ change due to brain-cell dam­age, or be­cause of “non-permanent” ef­fects due to dif­fer­ent en­vi­ron­ments.

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A study published last year that found a link between smoking marijuana and IQ decline may have been based on incomplete information, new research suggests. “The true effect could be zero” of pot on IQ, wrote Ole Rogeberg of the Ragnar Frisch Centre for Economic Research, Oslo, reporting the new findings in this week’s early online issue of the journal pnas. Alternatively, he added, there’s a chance pot might make you dumber, but indirectly—not through physical damage to brain cells, but through cultural factors. Rogeberg said his computer simulations showed that a different factor affecting IQ, socioeconomic status, could have skewed the previous results. He used the simulations to assess potential effects of socioeconomic status on IQ. The results indicated that this factor was enough to reproduce the association between IQ change and cannabis exposure reported in the previous study of a large sample of people from Dunedin, New Zealand, Rogeberg said. Previous publications based on the same Dunedin cohort indicated that early-onset pot smoking was more common in people with certain risk factors also tied to low socioeconomic status, he noted. Previous research, he added, also suggests that children with similar IQs but differing socioeconomic status wind up, for whatever reason, in, environments with different cognitive demands. That in turn causes their IQs to branch apart. “It would be too strong to say that the [previous] results have been discredited, but fair to say that the methodology is flawed and the causal inference drawn from the results premature,” he wrote. There is yet another interpretation of the Dunedin results, he added. Rather than pot damaging brain cells, it could be that “heavy, persistent, adolescent- onset cannabis use involves a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviors.” Further analyses of the Dunedin cohort could help distinguish between competing interpretations, he went on, and policy implications might differ depending on whether there is an IQ change due to brain-cell damage, or because of other “non-permanent” effects due to different environments. n