DNA was collected from 452 CDP participants from February 2006 to July 2007 via saliva samples using Oragene collection kits, under the supervision of a trained interviewer. Individuals received $20 and provided separate consent for their participation in the genotyping component of the project. DNA samples were labeled anonymously and sent to Washington University, St. Louis for extraction and genotyping. Genotyping for the MAOA-uVNTR locus was performed as in Caspi et al. 2002 with slight differences in primer sequence (available upon request). Alleles were categorized as “high activity” or “low activity” as in Sabol et al. 1998, i.e. alleles of 2, 3, or 5 repeats were categorized as “low” while those of 3.5 or 4 repeats were categorized as “high.” Because MAOA is an X-linked gene and unequivocal information about X-inactivation at this locus is not available, we limited our analyses to males. Furthermore, we conducted our analyses on only the subsample of European American males (maximum N=250) in an effort to minimize concerns about population stratification since allele frequencies are known to differ across ethnicities. DNA was available for
