[1] Univ London London Sch Hyg & Trop Med, Dept Social & Environm Hlth Res, London WC1E 7HT, England
[2] Penn State Univ, Dept Sociol, State Coll, PA USA
[3] Penn State Univ, Dept Anthropol, State Coll, PA USA
[4] Penn State Univ, Dept Demog, State Coll, PA USA