A new study published in the scientific journal Addiction found that the legalization of recreational cannabis in the United States appears to have caused an average increase of 20% in the frequency of cannabis use in those states. The study evaluated the effects of recreational cannabis legalization on a large sample of adult identical twins. Of particular interest were 111 pairs of identical twins in which one twin lived in a state with a different recreational cannabis policy than the other. The twins represent very well matched control groups and allow for a more accurate assessment of the causal impact of recreational legalization than studies of unrelated individuals. Looking at all study participants (1,425 people living in states with legal recreational cannabis use and 1,997 people living in states where recreational cannabis use is illegal), the study found about a 24% increase in the average frequency of cannabis use associated with legalization. Looking only at identical twins living in states with different policies, the twin living in the "legal" state was about 20% more likely to use cannabis than its counterpart living in the "illegal" state.  Cannabis is the most commonly used federally banned drug in the United States. Until 2014, cannabis could not be legally purchased or sold for recreational purposes anywhere in the United States. By early 2022, more than 141 million Americans lived in states where cannabis was legal for recreational purposes.