Kremen WS, Lyons MJ, Boake C, Xian H, Jacobson KC, Waterman B, Eisen SA, Goldberg J, Faraone SV, Tsuang MT. A discordant twin study of premorbid cognitive ability in schizophrenia. J Clin Exp Neuropsychol. 2006 Feb; 28(2):208-24.