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).
After his time at the Institute for Behavioral Genetics in Colorado, he moved to Pennsylvania State University where he studied elderly twins raised separately and together. He then joined the Institute of Psychiatry (King's College London) in 1994.