Does g variance change in adulthood? Testing the age de-differentiation hypothesis across sex Sergio Escorial*, Manuel Juan-Espinosa, Luis F. Garcı ´ a, Irene Rebollo, Roberto Colom Departamento de Psicologı´a Biolo ´gica y de la Salud, Facultad de Psicologı´a (Despacho 15), Universidad Auto ´noma de Madrid, 28049 Madrid Spain Received 27 December 2001; received in revised form 1 April 2002; accepted 27 April 2002 Abstract In the last decade, changes in the structure of intelligence across the life-span has become a central topic in the research on human intelligence. One of the main hypotheses that has arisen to account for such changes has been the age de-differentiation hypothesis [Balinsky, Genetic Psychology Monographs 23 1941, 191]. It predicts an increase in the importance of g, and a decrease in the number and importance of the lower-order abilities from early maturity to senescence. Despite of the research effort to test this hypothesis, no study has ever been conducted controlling by sex. For that purpose, males and females of the Spanish standardisation sample of the WAIS-III were analysed separately. Results show that the importance of g does not change with age irrespective of sex. Thus, the age de-differentiation hypothesis is rejected for both males and females. The indifferentiation hypothesis is supported as a more appropriate view of the changes in the structure of intelligence across adulthood. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Age de-differentiation hypothesis; Structure of intelligence; Adulthood; Senescence; Sex; g Factor; Indiffer- entiation hypothesis; WAIS-III 1. Introduction A substantial percentage of variance in human mental ability can be accounted for by a general intelligence factor (Carroll, 1993; Jensen, 1998; Spearman, 1927). The age-differentiationhypothesis, as coined by Garrett (1946), predicts a decrease in the variance accounted for by g from childhood to 0191-8869/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-8869(02)00133-2 Personality and Individual Differences 34 (2003) 1525–1532 www.elsevier.com/locate/paid * Corresponding author. Tel.: +3491-397-51-83; fax: +3491-397-52-15. E-mail address: sergio.escorial@adi.uam.es (S. Escorial).