He then worked at the Institute for Behavioral Genetics at the University of Colorado Boulder. From 1986 until 1994 he worked at Pennsylvania State University, studying elderly twins reared apart and twins reared together to study aging and since 1994 has been at the Institute of Psychiatry (King's College London).
He worked at the Institute for Behavioral Genetics at the University of Colorado Boulder from 1986 to 1994, he continued his research on elderly twins raised apart and together at Pennsylvania State University, and then joined the Institute of Psychiatry (King's College London) in 1994.