A new study from Norway says that poverty rather than marijuana use may be behind the decreased cognitive abilities of some heavy marijuana users.

Dr Ole Rogeberg, economist and researcher from Ragnar Frisch Centre for Economic Research in Oslo, says that the declines in cognitive functioning of teens using marijuana may be explained by socioeconomic factors, rather than a direct effect of marijuana use. The study from New Zealand that was published last year and included more than 1,000 teens has shown that use of cannabis before the age of 18 could lower intelligence.

Rogeberg's study was based on simulation models to analyze the role of marijuana use on teens' brain. He argues that the initial study failed to study the effects of poverty on teens that use marijuana.

"Recent research indicates that IQ and brainpower are kind of like muscular strength: strengthened if it is regularly challenged. IQ is strengthened or sustained by taking education, studying hard, spending time with smart, challenging people, doing demanding work in our jobs. Some kids, unfortunately, are burdened with a poor home environment, poor self-control and conduct problems. These kids are likely to gradually shift away from the kinds of activities and environments that would exercise their IQs," Rogeberg told HealthDay.

Authors of the initial study have issued a joint statement saying that the latest study by Rogeberg isn't supported by their study data.

"While Dr. Rogeberg's ideas are interesting, they are not supported by our data," wrote study authors Terrie Moffitt, Avshalom Caspi and Madeline Meier from Duke University. The authors added that about 23 percent of their study participants came from lower socioeconomic class.

Other experts have said that because the two studies have a different approach to analyze the effects of marijuana use, their study results can't be compared.

"The original paper is led by authors specialising in psychology and psychiatry. They do not consider variables such as socioeconomic status, and their statistical analyses rely heavily on strong and untested assumptions," said Annette Dobson, director at the centre for longitudinal lifecourse research at the University of Queensland."In contrast, Dr Rogeberg, an economist...takes socio-economic status into account, and has a more thorough approach to the statistical methods he applies. He is also clearer about the uncertainty of his results, using simulations to explore the potential effects of different assumptions."

The study has been published in the journal PNAS.