The participants, many of whom were born in Colorado and Minnesota but had since relocated, were surveyed pre- and post-2014 on how many days they used cannabis in the last six months, and scientists initially found there was about a 24% increase in usage in states that legalized recreational cannabis compared with ones that did not. Based on where respondents were living at the time of the surveys, nearly every state was represented, along with Washington, DC and Puerto Rico.
The study included 111 pairs of identical twins as well, with one twin living in a state that legalized recreational cannabis and the other in a state that did not legalize it. Among the identical twins, researchers found the usage increased about 20% in states that legalized recreational cannabis compared with ones that did not.
Since identical twins share so many similarities, that percentage is a more accurate estimate of the causal influence of cannabis legalization on cannabis use, said lead study author Stephanie Zellers, now a researcher at the University of Helsinki in Finland.
Identical twins share the same genes and often the same type of upbringing, both of which could influence how frequently someone uses cannabis, said Zellers, who was a doctoral candidate in psychology at the University of Minnesota at the time of the project.