[1] PENN STATE UNIV, CTR SPECIAL POPULAT & HLTH, UNIVERSITY PK, PA 16802 USA
[2] PENN STATE UNIV, CTR DEV & HLTH GENET, UNIVERSITY PK, PA 16802 USA
[3] ST THOMAS HOSP, TWIN RES UNIT, LONDON, ENGLAND
[4] VRIJE UNIV AMSTERDAM, DEPT PSYCHOL, AMSTERDAM, NETHERLANDS
[5] LEIDEN UNIV, DEPT HUMAN GENET, NL-2300 RA LEIDEN, NETHERLANDS
[6] TNO, INST PREVENT & HLTH, LEIDEN, NETHERLANDS