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.

Although I find Blair's case for arguing for the distinction between fluid cognitive functions and general intelligence less than compelling, I believe him. However, I also believe that what is required next is a theory of both general intelligence and fluid cognitive functions that articulates the distinction. In the absence of this, more data, particularly of the neuroscience variety, is likely to stall rather than advance progress.

In agreement with Blair, I favor the idea of dissociative patterns in cognitive performance, even more when it comes to development. However, such dissociations are present not only between fluid cognition and general intelligence, but also within fluid cognition itself. Heterogeneity of executive attention, even when indexed with a single paradigm, is further discussed in relation to anterior cingulate cortex.

Blair's account, like the intelligence field in general, treats many distinct constructs as if they were practically interchangeable – this is not self-evident. Paradigm integration and rationalization of redundant nomenclature are important for the continued development of understanding. The prior task is to demonstrate where synonymity of constructs across paradigms occurs, and where it fails. We present arguments why this is the case.

Blair proposes that fluid intelligence, working memory, and executive function form a unitary construct: fluid cognition. Recently, our group has utilized a combined correlational–experimental cognitive neuroscience approach, which we argue is beneficial for investigating relationships among these individual differences in terms of neural mechanisms underlying them. Our data do not completely support Blair's strong position.

Blair describes fluid cognition as highly related to working memory and executive processes, and dependent on the integrity of frontal-lobe functioning. However, the literature review appears to neglect potential contributions to fluid cognition of the focus of attention as an important information-storage device, and the role of posterior brain regions in that kind of storage. Relevant cognitive and imaging studies are discussed.

Two studies substantiating Blair's main postulates are summarized. The first study showed that fluid cognition, reasoning, and perceived competence about reasoning are separate and equipotent partners in g. The second study showed that reasoning, understanding of emotions, and perceived competence about reasoning and emotions partake in the formation of g, substantiating Blair's claim that cognition and emotion are linked in the brain.

Brain physiology and IQ gains over time both show that various cognitive skills, such as on-the-spot problem solving and arithmetic reasoning, are functionally independent, despite being bundled up in the correlational matrix called g. We need a theory of intelligence that treats the physiology and sociology of intelligence as having integrity equal to the psychology of individual differences.

From the stance of cognitive developmental theories, claims that general g is an entity of the mind are compatible with notions about domain-general development and age-invariant individual differences. Whether executive function is equated with general g or fluid g, research into the mechanisms by which development occurs is essential to elucidate the kinds of environmental inputs that engender effective intervention.

Although working memory capacity and executive function contribute to human intelligence, we question whether there is an equivalence between them and fluid intelligence. We contend that any satisfactory neurobiological explanation of fluid intelligence needs to include abstraction as an important computational component of brain processing.

Blair equates the constructs of working memory (WM), executive function, and general fluid intelligence (gF). We argue that there is good reason not to equate these constructs. We view WM and gF as separable but highly related, and suggest that the mechanism behind the relationship is controlled attention – an ability that is dependent on normal functioning of the prefrontal cortex.

Blair presumes the validity of the fluid-crystallized model throughout his article. Two comparative evaluations recently demonstrated that this presumption can be challenged. The fluid-crystallized model offers little to the understanding of the structural manifestation of general intelligence and other more specific abilities. It obscures important issues involving the distinction of pervasive learning disabilities (low general intelligence) from specific, content-related disabilities that impede the development of particular skills.

Blair makes a strong case that fluid cognition and psychometric g are not identical constructs. However, he fails to mention the development of the prefrontal cortex, which likely makes the Gf–g distinction different in children than in adults.1 He also incorrectly states that current IQ tests do not measure Gf; we discuss several recent instruments that measure Gf quite well.

We welcome Blair's argument that the relationship between fluid cognition and other aspects of intelligence should be an important focus of research, but are less convinced by his arguments that fluid intelligence is dissociable from general intelligence. This is due to confusions between (a) crystallized skills and g, and (b) universal and differential constructs.

The dissociation of fluid cognitive functions from g is implicit in the Cattell-Horn-Carroll gF-gC theory. Nevertheless, Blair is right that fluid functions are extremely important. I suggest that the key mental operation assessed by measures of gF is the ability to sustain mental simulation while keeping the relevant representations decoupled from the actual world – an ability that underlies all hypothetical thinking.

I propose a neuroscience and animat research-inspired model and a thought experiment to test the hypothesis of a developmental relation between fluid and crystallized intelligence. I propose that crystallized intelligence is the result of well-defined activities and structures, whereas fluid intelligence is the physiological catalytic adaptation mechanism responsible for coordinating and regulating the crystallized structures. We can design experiments to reproduce exemplified normal and anomalous phenomena, especially disorders, and study possible cognitive treatments.

Blair argues that fluid cognition is dissociable from general intelligence. We suggest that a more complete understanding of this dissociation requires development of specific process models of the mechanisms underlying fluid cognition. Recent evidence indicates that relational integration and inhibitory control, both dependent on prefrontal cortex, are key component processes in tasks that require fluid cognition.

Blair's assertion that fluid intelligence (gF) is distinct from general intelligence (g) is contradictory to cumulative evidence from intelligence research, including extant and novel evidence about generational IQ gains (Lynn–Flynn effect). Because of the near unity of gF and g, his hypothetical concept of gF' (gF “purged” of g variance) may well be a phlogiston theory.

Blair boldly proposes a model integrating different aspects of intelligence. Its real-life value can be put to the test by using programs designed to develop children's abilities in areas predicted to be crucial for minimizing adverse outcome. Until support from such programs is available, the model is an interesting hypothesis, albeit with remarkable possible repercussions. As such, it seems worthy of further development.

The question of whether fluid intelligence can be differentiated from general intelligence in older adults is addressed. Data indicate that the developmental pattern of performance on fluid tasks differs from the pattern of general intelligence. These results suggest that it is important to identify changes in fluid cognitive functions associated with frontal lobe decline, as they may be early indicators of cognitive decline.