Is there a way to find out the answer definitively?
Perhaps — by comparing the Dunedin Study with another in a different country. Such comparisons have been done before. For example, the United Kingdom's Avon Longitudinal Study of Parents and Children (ALSPAC) found that children who were breastfed for longer went on to have higher IQs, lower blood pressure and lower body mass indices than those who were not. However, longer breastfeeding is associated with higher socioeconomic status in the United Kingdom; when the data were compared with data from the Pelotas longitudinal study in Brazil, where breastfeeding is not associated with higher socioeconomic status, the link with increased IQ was maintained, but the other benefits disappeared.
What do other scientists think?
Mitch Earleywine, a psychologist at the University at Albany, State University of New York, says that Røgeberg's analysis definitely supports the idea that links between adolescent cannabis use and drops in IQ are essentially spurious, arising from socioeconomic differences rather than any sort of pharmacological action. John Macleod of the University of Bristol, UK, who works on the ALSPAC data, points out that Meier and her colleagues acknowledged in their original paper that the results might be caused by confounding factors. He adds that the modeling in Røgeberg's paper shows that within a set of reasonable assumptions, this is indeed possible.