Figure 1.

The sibling difference in delayed verbal memory, based on sibling differences in (A) age of onset of monthly use and (B) frequency of use in the past six months. Error bars represent standard errors around the point estimate. For age of onset (A), using cannabis two or more years earlier than one’s sibling was associated with a 0.40 standard-deviation decrease in delayed verbal memory performance, relative to the mean performance of participants from the same family (i.e., after accounting for familial factors shared by siblings).For frequency of use (B), using cannabis two or more days per week than one’s sibling was associated with a 0.23 standard-deviation decrase in delayed veriabl memory performance, relative to the mean performance of participants from the same family (i.e., after accounting for familial factors).