The familial association between cigarette smoking and ADHD: a study of clinically referred girls with and without ADHD, and their families.

Attention-deficit/hyperactivity disorder (ADHD) is a strong risk factor for smoking. Since both ADHD and smoking are familial disorders, one way to further our understanding of this association is to examine the familial relationship between them. Our aim was to evaluate the familial association between ADHD and smoking in families ascertained from girls with and without ADHD. Subjects were derived from a longitudinal case-control family study of girls with (n = 140) and without (n = 122) ADHD ascertained from pediatric and psychiatric clinics, and their biological first-degree relatives. Diagnoses of ADHD and smoking (i.e., full or subthreshold nicotine dependence) were made with structured psychiatric interviews. We stratified the relatives into four groups based on probands' ADHD and smoking status: (1) relatives of controls without smoking (probands n = 100, relatives n = 317), (2) relatives of controls with smoking (probands n = 22, relatives n = 71), (3) relatives of ADHD girls without smoking (probands n = 100, relatives n = 320), and (4) relatives of ADHD girls with smoking (probands n = 39, relatives n = 133). We compared the rates of ADHD and smoking across the four relative groups using survival analysis. ADHD in the proband increased the risk for ADHD in the relatives irrespective of probands' smoking and smoking in the proband increased the risk for smoking in the relatives irrespective of probands' ADHD status. Furthermore, we found statistically significant evidence for cosegregation of smoking and ADHD, suggesting that the two disorders are transmitted together through families more often than expected by chance. These findings support the hypothesis that the combination of ADHD and smoking comprises an etiologically distinct familial subtype of ADHD in girls.