1 Division of Psychiatry, University College London, London W1T 7NF, UK. Electronic address: g.livingston@ucl.ac.uk.
2 Division of Psychiatry, University College London, London W1T 7NF, UK.
3 Department of Neurology and Department of Psychiatry and the Behavioural Sciences, Keck School of Medicine, Leonard Davis School of Gerontology of the University of Southern California, Los Angeles, CA, USA.