Edinburgh Research Explorer Clarifying process versus structure in human intelligence: Stop talking about fluid and crystallized. Citation for published version: Johnson, W & Gottesman, II 2006, 'Clarifying process versus structure in human intelligence: Stop talking about fluid and crystallized.', Behavioral and Brain Sciences, vol. 29, no. 2, pp. 136-137. https://doi.org/10.1017/S0140525X06329032 Digital Object Identifier (DOI): 10.1017/S0140525X06329032 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Behavioral and Brain Sciences Publisher Rights Statement: ©Johnson, W., & Gottesman, I. I. (2006). 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Oct. 2021 BEHAVIORAL AND BRAIN SCIENCES (2006) 29, 109–160 Printed in the United States of America How similar are fluid cognition and general intelligence? A developmental neuroscience perspective on fluid cognition as an aspect of human cognitive ability Clancy Blair Human Development and Family Studies, Pennsylvania State University, University Park, PA 16802-6504 [email protected] Abstract: This target article considers the relation of fluid cognitive functioning to general intelligence. A neurobiological model differentiating working memory/executive function cognitive processes of the prefrontal cortex from aspects of psychometrically defined general intelligence is presented. Work examining the rise in mean intelligence-test performance between normative cohorts, the neuropsychology and neuroscience of cognitive function in typically and atypically developing human populations, and stress, brain development, and corticolimbic connectivity in human and nonhuman animal models is reviewed and found to provide evidence of mechanisms through which early experience affects the development of an aspect of cognition closely related to, but distinct from, general intelligence. Particular emphasis is placed on the role of emotion in fluid cognition and on research indicating fluid cognitive deficits associated with early hippocampal pathology and with dysregulation of the hypothalamic-pituitary-adrenal axis stress-response system. Findings are seen to be consistent with the idea of an independent fluid cognitive construct and to assist with the interpretation of findings from the study of early compensatory education for children facing psychosocial adversity and from behavior genetic research on intelligence. It is concluded that ongoing development of neurobiologically grounded measures of fluid cognitive skills appropriate for young children will play a key role in understanding early mental development and the adaptive success to which it is related, particularly for young children facing social and economic disadvantage. Specifically, in the evaluation of the efficacy of compensatory education efforts such as Head Start and the readiness for school of children from diverse backgrounds, it is important to distinguish fluid cognition from psychometrically defined general intelligence. Keywords: cognition; cognition-emotion reciprocity; developmental disorders; emotion; fluid cognition; Flynn effect; general intelligence; limbic system; neuroscience; phenylketonuria; prefrontal cortex; psychometrics; schizophrenia 1. Introduction biological bases of g, and the extent to which g is itself reducible to known, theoretically tractable cognitive pro- 1.1. What is general intelligence? cesses, are interrelated, overarching questions high on Historically, theories of intelligence have focused on the the priority list for intelligence researchers. identification of a single factor, referred to as psychometric As a mathematically defined entity with large expla- g, that has been shown to underlie performance on tests of natory power, the general factor has been pervasive mental abilities (Spearman 1927). The single-factor theory in the psychological literature for over one hundred reflects the fact that the various subtests of IQ measures years. However, it is important to keep in mind that correlate positively. Although several alternative interpret- ations exist as to just what this positive manifold among tests means, it is beyond question that a single mathemat- CLANCY BLAIR, Ph.D., M.P.H., is an Associate ically derived factor can be extracted from tests of diverse Professor in the Department of Human Development mental abilities (Carroll 1993; Jensen 1998; see the edited and Family Studies at Pennsylvania State University. volume by Sternberg & Grigorenko 2002). Jensen (1998) He is the author of numerous peer-reviewed pub- provides a comprehensive review of research on g, detail- lications examining the effects of early psychosocial ing the evidence for g and the relation of g to various real- adversity on child health and development. His chief interest concerns the interrelation of cognitive and world outcomes by using the method of correlated vectors. emotional development early in the life course, and There are, however, numerous questions about g that are the ways in which neurobiology and experience interact the subject of ongoing research and scientific exchange. In to shape developmental trajectories in early childhood. particular, questions about the unitary nature of g, the # 2006 Cambridge University Press 0140-525x/06 $12.50 109 Blair: How similar are fluid cognition and general intelligence? psychometric g is not a thing in itself but a manifestation of two aspects of functioning covary to an extent that they some as yet undefined properties of brain structure and appear to be unitary. Overall, the available evidence function. In fact, it is only in the last several decades suggests that fluid cognition is an aspect of cognitive func- that the underlying processes and functions through tioning that can be under considerable environmental which g may be manifest have begun to be worked out influence both cumulatively over time and interactively in any detail, albeit with only limited success. For within context in a way that indicates it to be a highly example, a general speed-of-processing hypothesis has salient influence on behavior, but one that is distinct been examined using measures of inspection time and from general intelligence, psychometrically defined. reaction time. Although the conceptual relation of these Several sources of evidence indicate dissociation of fluid measures to g is sound, a number of studies have shown cognition from g, and the purpose of this target article is to that their empirical relation to g tends to be small review this evidence and to consider its implications for (Crawford et al. 1998; Luciano et al. 2004). Similarly, a understanding cognition and human behavior. The first conceptually sound general synaptic plasticity thesis has source of evidence is psychometric and at the population been proposed as an underlying basis for g (Garlick level of analysis and concerns the worldwide rise in the 2002). However, although perhaps promising, the synaptic mean level of mental test performance known as the plasticity argument lacks specificity as to a clear relation Flynn effect. The second is neuroscientific and at the indi- between brain plasticity and intelligence and at present vidual level and concerns evidence indicating fluid cogni- offers no way of measuring individual differences in tive and learning impairments in humans and in animals plasticity that might correlate with psychometric g. with damage to a neural network integrating areas of the In apparent contrast to general speediness and plasticity prefrontal cortex with structures of the brain’s limbic explanations for g, work on fluid cognitive functions – region. A third source of evidence is neuropsychological those associated with general reasoning and problem- and concerns the extent to which cognitive impairments solving processes and referred to primarily as working in identified developmental disorders are consistent with memory (WM), executive function (EF), or fluid intelli- a pattern of dissociation between fluid cognitive functions gence (gF) – seems to have shown some early and substan- and general intelligence. Having considered this evidence, tial promise in indicating that both the conceptual basis work examining reciprocal interconnectivity among limbic and neural structural basis for g may be close at hand. brain structures associated with emotional reactivity and However, as is argued in section 3 and the sections that the stress response and prefrontal cortical structures follow, this promise is perhaps more apparent than real. associated with fluid cognition is reviewed. It is argued Given psychometric evidence for a relation between that this work indicates potentially large environmental fluid cognition