Our initial sample of 567 individuals was comprised of 398 heroin addicts and 169 controls, with a mean age of 26–31 years, as shown in Table 1. The participants were predominantly northern Han Chinese from Sichuan Province; these are a more homogeneous population than the southern Han. Chinese ethnic minorities were not included in the sample to reduce the likelihood degree of population stratification. The cases were recruited from inpatient clinics attached to two psychiatric hospitals in Chengdu (Southwest China) and were diagnosed with DSM-IV criteria for heroin dependence using a semi-structured clinical interview. This interview included questions on (i) age at first use of heroin and duration of use, (ii) the quantity of drug consumed over this period, (iii) whether the patient had abused other addictive substances such as alcohol, cocaine, cannabis, etc., (iv) co-morbidity of other psychiatric conditions. Those subjects who were abusing other substances or suffered from a major psychiatric illness such as recurrent major depression, schizophrenia or bipolar disorder were not included in the study. To corroborate patient interview, case notes were examined and a family
