Familial influences on substance use by adolescents and young adults.

This study uses data from the Ontario Health Survey to examine within-family influences (sibship number, age and sex composition; family structure and parental substance use) on the use of tobacco, alcohol and marijuana in households (N = 4,643) among offspring aged 12 to 24 years. Using a modification of the kappa statistic, concordance among siblings is modest generally and undifferentiated across substance type. Concordance is stronger among sibships that are either all male or older (19-24 years) and is particularly strong for siblings < or = two years apart in age. The dominant influence of substance use behaviour appears to be from older siblings to younger siblings and not from parents to offspring. Sibling concordance for substance use suggests that the treatment and prevention of substance use (and abuse) among adolescents and young adults might be enhanced by including a family focus, especially where there are two or more siblings at home.