[1] Duke Univ, Sanford Sch Publ Policy, Box 90239, Durham, NC 27708 USA
[2] RTI Int, 3040 East Cornwallis Rd, Res Triangle Pk, NC 27709 USA
[3] Penn State Univ, Methodol Ctr, 431 Hlth & Human Dev Bldg, University Pk, PA 16802 USA
[4] Kings Coll London, MRC Social Genet & Dev Psychiat SGDP Ctr, Dept Child & Adolescent Psychiat, Inst Psychiat Psychol & Neurosci, 16 De Crespigny Pk, London SE5 8AF, England
[5] Duke Univ, Ctr Dev Epidemiol, 905 W Main St,Suite 228 Brightleaf Sq, Durham, NC 27701 USA
[6] Duke Univ, Dept Psychol & Neurosci, 417 Chapel Dr, Durham, NC 27708 USA