Outlook of the academicians and the laity on the issue of whether intelligence is a single ability or multiple abilities seems to be in contrast with each other. Moreover, there is still some ongoing debate surrounding the generally accepted view among academic psychologists about the existence of a general intelligence rather than different specific abilities which are not closely related. This article will attempt to comprehensively and briefly review the existing literature pertaining to the aforementioned topic of general intelligence which seems to be a single ability and its criticism with the emergence of theories of multiple intelligence.
Finally, it is discussed why the ‘different forms of intelligence’ are, in fact, specific skills and why theory of multiple intelligences does not hold water when analyzed through the lens of advanced neuroimaging techniques.Charles Spearman was one early supporter of the notion of intelligence being a single characteristic. In his research done on school children, he observed that there is a significant positive correlation among student’s performance on relatively unrelated subjects like classics, mathematics and music (Spearman, 1904). He argued that this correlation indicates an underlying general mental faculty, which he termed as ‘g’ factor, that taps into performance in all of the subjects. He believed that apart from a primary g factor, one or more specific or ‘s’ factors relating to particular tasks also exist.
Spearman expressed less interest in studying s factors as he believed them to be less influential than g in determining performance level. According to him, the general theorem could be stated as “Whenever branches of intellectual activity are at all dissimilar, then their correlations with one another appear wholly due to their being all variously saturated with some common fundamental Function (or group of Functions)” (Spearman, 1904).According to John B. Carroll, the g factor of intelligence encompasses more than sixty individual cognitive abilities in one common factor (Colom, Jung, & Haier, 2006). Dr. Adrian Furnham argues that ‘most academic psychologists are ‘g’ men believing the extensive available evidence points to the fact that people tend to score similarly on very different tests’ (Furnham, Classics French English Mathematics2015).
The contemporary widely accepted g-centered theory of cognitive abilities is the Cattell – Horn – Carroll Theory (CHC Theory) which combines the Cattell’s theory of fluid and crystallized intelligence (1963) with John B. Carroll’s Three Stratum Theory of cognitive abilities (1993). The theory suggests that intelligence is conceptualized in a hierarchy of three stratums or levels of narrow, broad and general abilities (McGrew, 2008).Traditional intelligence tests or IQ tests like Stanford-Binet, Raven’s Progressive Matrices and Wechsler Adult Intelligence Scale (WAIS) are designed to measure the g (Gregory, 2004).
The credibility of g is also corroborated by the fact that significant correlations on the order of .30 to .40 have often been found between various measures of processing speed i.e. the speed with which individuals perform simple perceptual and cognitive tasks and scores on intelligence tests (Baron, 2011).
In two studies (N=85 & N= 88) investigating the relationship among measures of intelligence, processing speed and peripheral nerve conduction velocity (NRV), significant correlations of .42 and .48 were found between NCV and IQ scores (Peterson, 2017). According to Dr. Jordan Peterson, the results are interpreted in terms of a ‘neural efficiency’ model of intelligence, which has received support from other studies of physiological correlates of human intelligence. Furthermore, the g seems to have biological origin in that it is related with the volume of gray matter in the brain.
In one study, structural magnetic resonance imaging and voxel-based morphometry was used to investigate the relationship between individual differences in the amount of regional gray matter volumes across the entire brain. It was found that gray matter volumes across the entire brain were correlated with eight cognitive tests showing distinguishable g involvement (Colom, Jung, & Haier, 2006). Moreover, multivariate genetic analyses suggest that there is genetic overlap among cognitive abilities i.e. it is likely that same gene/genes are associated with different cognitive abilities like verbal and spatial ability (Plomin & Petrill, 1997).
Despite robust empirical support which includes IQ scores and performance, physiological correlates, and neuroimaging, the conceptualization of intelligence as single ability is not without its criticism. According to Cooper, “many researchers and lay people have been dissatisfied with a traditional, ‘narrow’ conceptualization of intelligence, which emphasizes verbal and performance IQ and other more ‘academic’ abilities.” Some researchers have promoted the idea that our notion of intelligence should be expanded to include other different forms of intelligence as well. These researchers believe that the faculty of intelligence is composed of many different abilities which work independently of one another.
One such prominent theory, apart from Thurstone’s theory which proposed seven different factors and Sternberg’s Triarchic theory, was given by Howard Gardner in 1983 and it was called Multiple Intelligence theory (MI theory). His chief objection to the traditional g-centered view of intelligence was that the researchers focused only on ‘normal’ people and failed to take into account the achievements of those at the extreme ends. According to him, more comprehensive view of intelligence could be generated by studying individuals not only in the middle but also those who are at the superior end of the spectrum and those who are cognitively impaired (Baron, 2011).
Gardener proposed eight different types of intelligences which include interpersonal, intrapersonal, naturalist, musical, linguistic/verbal, analytical/mathematical, visuospatial and body-kinesthetic. He argued that these intelligences are more or less independent of each other and an individual can score high on one type of intelligence while low on some other.This notion that there are multiple independent intelligences is appealing because it supports the general egalitarian sentiment by indicating that everyone is ‘intelligent’ in one way or the other. Here, it should be noted that the primary objective of any scientific endeavor has never been about making people feel good about themselves. In this context, there are two serious problems with this theory of ‘multiple intelligences’: one is definitional problem and another one is empirical problem. The advocates of MI theory define intelligence in a very unscientific and messy way.
Gardner (1983) defined intelligence as “the ability to solve” problems or to create products that are valued within one or more cultural settings.” Is ‘ability to create products that are valued within one/more cultural settings’ an intelligence or a specific skill? You cannot just call anything you can measure (here the ability to make a valuable product) as intelligence. To solve this definitional problem of intelligence, researchers Shane Legg and Marcus Hutter analyzed over 70 definitions of intelligence given by eminent individual psychologists, Artificial Intelligence researchers and organizations like APA and identified its three components that were common in virtually all the definitions. By combining these three components they gave a standard definition of intelligence as “intelligence measures an individual’s ability to achieve goals in a wide range of environments” (Legg & Hutter, 2006), (Legg & Hutter, A Collection of Definitions of Intelligence, 2006). When the MI theory is scrutinized using this definition, it is patently clear that Gardner’s ‘intelligences’ are in fact specific skills as they do not necessarily facilitate an individual’s success in a wide variety of environments.
There are empirical problems with MI theory as well. According to one study in which factor analysis was done, the nine intelligences were found to be highly intercorrelated with each other (McGreal, 2013). Essentially, what this means is that Gardner’s claim that these intelligences are independent is wrong.
In 1999, Gardner asserted that MI theory depends on each intelligence having its own neural processing circuit, arguing that if “musical and spatial processing were identically represented” in neural circuits “that fact would suggest the presence of one intelligence, and not two separate intelligences.” Evidence was indeed found out showing shared neural circuits for the processing of many different types of content (Waterhouse, 2006) further corroborating the fact that Gardner’s intelligences are not independent and do not represent different forms of intelligence.However, it should be noted that the issue is far from being settled. Though the g-centered models of intelligence enjoy broad and robust empirical support, it is possible that they have errors of omission and commission. These gaps, if any, need to be filled by further research while simultaneously keeping our minds open to the possibility that conceptualization of intelligence as multiple abilities might not be entirely wrong.