Fluid intelligence and neural efficiency: effects of task complexity and sex Aljoscha C. Neubauer*, Andreas Fink Institute of Psychology, University of Graz, Universita ¨tsplatz 2, Graz 8010, Austria Received 11 March 2002; received in revised form 5 August 2002; accepted 10 September 2002 Abstract On the basis of the neuralefficiency concept of human intelligence—which suggests a more efficient use of the cortex (or even the brain) in brighter individuals—we analyzed the role of sex and task complexity as possible moderating variables in the relationship between cortical activation and psychometric intelligence. In 28 males and 30 females we measured cortical activation in the EEG during performance of a modified variant of Stankov’s Triplet Numbers test—a test consisting of five increasingly complex conditions. Preliminary analyses showed that both sex- and intelligence-related effects on cortical activation became more evident when we focused on participants’ fluid intelligence scores instead of just using scores of general mental ability. As expected, we observed an increase of cortical activation with increasing task complexity, but no moderating influence of task complexity on the intelligence-activation relationship was found. Moreover, our data suggest that the males were more likely to produce cortical activation patterns in line with the neural efficiency hypothesis (i.e. less activation in brighter individuals), whereas in females no significant differences were found. # 2002 Elsevier Ltd. All rights reserved. Keywords: Intelligence; Fluid intelligence; Complexity; Reaction times; EEG; Event-related desynchronisation; Sex differences; Neural efficiency Neuroimaging techniques like the measurement of the brain’s glucose metabolism rate (GMR) using positron emission tomography (PET) have produced empirical evidence of a more strongly localized cortical activation during cognitive task performance in brighter as compared to less intelligent individuals (Haier, Siegel, MacLachlan et al., 1992; Haier et al., 1988; Haier, Siegel, Tang, Abel, & Buchsbaum, 1992; Parks et al., 1988). This more strongly focused cortical activa- tion (resulting in a lower total cortical activation) in brighter as compared to less intelligent individuals is often interpreted within the so-called neural efficiency hypothesis that suggests a 0191-8869/03/$ - see front matter # 2002 Elsevier Ltd. All rights reserved. PII: S0191-8869(02)00285-4 Personality and Individual Differences 35 (2003) 811–827 www.elsevier.com/locate/paid * Corresponding author. Tel.: +43-316-380-5124; fax: +43-316-380-9811. E-mail address: aljoscha.neubauer@kfunigraz.ac.at (A.C. Neubauer).