The central argument of Jensen's (1998) book is that the g-factor is a biological phenomenon. Thus, individual and population differences are a function of evolutionary processes. Several key assumptions underlying this assertion are critically considered here.
2. On a surface level there are several problems with the empirical evidence Jensen uses to demonstrate that g is a biological phenomenon. It is commonly accepted that brain structure and functional organization are related to the production of complex cognitive behaviors, such as intelligence. Several of the indices reported by Jensen are of questionable validity, however. For example, correlations between head circumference or global brain volume and intelligence are unreliably reported in the literature. Head circumference is such a gross measure of cranial capacity that it is hardly useful. Intracranial endocasts are often used to determine the cognitive capabilities of early hominids; however, this is due primarily to the lack of more accurate measures rather than the validity of the method.
3. More accurate measures of brain volume assessed by MRI techniques produce very small, if significant, relationships with intelligence. Indeed there is ample evidence that simple measures of brain size are of little utility in understanding individual differences in "intelligence". More reliable measures of brain organization related to cognitive capabilities are the encephalization quotient (the ratio of brain size to body size) and the neocortical ratio (the ratio of neocortex to total brain size). Such measures have shown more consistent relationships with complex cognitive abilities such as language and symbolic reasoning in comparative studies (Deacon, 1997; Jerison1973 Rumbaugh, 1997). However, there is no evidence for meaningful intraspecies differences relating these measures to cognitive behavior. Every aspect of ostensible empirical evidence reported by Jensen as supporting the biological nature of g appears to be plagued by similar overstatements and misinterpretations.
4. Jensen extends his biometric argument even further by arguing that these purported biological correlates are rooted in genetics. Thus, ultimately intelligence is a function of genotype and inter-group difference can be attributed to genetics. The racial arguments given here are not worthy of comment. The relationship between genetics and intelligence has been amply disputed elsewhere (e.g., Capron, Vetta, Duyme & Vetta, 1999; Hirsch, 1997; Roubertoux & Capron, 1990).
5. It appears that Jensen's primary reason (leaving aside for the moment justifications for racial disparity) for trying so hard to find a biological foundation for general intelligence is that if it were a biological phenomenon then it would be a real phenomenon. Jensen claims that having physiological correlates proves that g is not just an artifact of factor analytic methodology, but rather a biological phenomenon. Disregarding that this just seems to be a gross logical misstatement, there is an implicit assumption here that is quite troubling. Jensen seems to be arguing that biological phenomena are more "real" than behavioral phenomena. Since the primary focus of psychology is the study of behavior, and behavior is a process, it can be argued that psychology is a process-oriented science. That the observational properties of processes differ from those of entities does not necessarily imply that one is more valid than another. That a behavioral process, such as intelligence, is not isomorphic with a biological structure, does not imply that it is invalid.
6. Perhaps Jensen misunderstands the ontological relationships between structure and process, for he implicitly assumes a strong reductionism in relating biology to behavior. In relating biological factors to complex behaviors, of which a second order latent factor like Spearman's g is certainly an example, a model of strong reductionism simply does not work. A much more coherent model is found in the concept of integrative levels (Greenberg, Partridge, Weiss, and Harroway, 1999). From this perspective, neural organization and function is one of a number of integrated factors that jointly influence each other. For example, increased neural complexity allows for more complex environmental interactions, which in turn lead to increases in neural complexity. These processes function in a probabilistic epigenetic manner and cannot be reductively deconstructed without significant loss of information. Thus, from this perspective the statement that g is a biological phenomenon is without meaning. A more coherent statement is that g is a feature of a dynamic organism-context system.
Capron, C., Vetta, A. R., Duyme, M., & Vetta, A.. (1999) Misconceptions of biometrical IQists. Cahiers de Psychologie Cognitive, 18, 115-160.
Deacon, T. W. (1997). The symbolic species: The co-evolution of language and the brain. New York: Norton.
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