As previously discussed, this developmental shift may suggest an increased salience in environmental factors that are shared by twins such as peers, neighborhood effects (e.g., drug availability), and degree of parental supervision. We recognize, however, that the hypothesized developmental changes from age 12 to age 17 cannot be disentangled from the possible effects of using slightly different measures at the two time points. Although we used age-appropriate measures of substance use and novelty seeking, they were not completely isomorphic (however, ADHD and conduct disorder composites came from the same instruments at both waves). Though justified, this methodological inconsistency should be considered when interpreting differences in the results across the two ages.
