Article summary with Fluid intelligence: A brief history by Kent - 2017

Approximately 75 years ago, the concept of fluid and crystallised intelligence was first introduced to the psychological community by Raymond B Catell. The specification of the structure of intelligence has been considered the greatest achievement in psychology. Indeed, a consensus has emerged regarding the taxonomy of intellectual abilities. Nowadays, the concept of intelligence continues to be a topic of active research and controversy. The present article aims to provide a brief overview of the origin of this concept, the early efforts to define intelligence and uses of intelligence tests to study pressing social issues, and the ongoing controversies with fluid intelligence and the structure of intelligence. 

How has the concept of intelligence developed during the nineteenth century? 

The first scientific study into the nature of intellectual functioning dates back to Francis Galton (1883) in the late nineteenth century. Galton used a quantitative index of relationship called correlation to study the nature of basic human abilities. 

Following the spirit of Galton, James McKeen Catell (1890) proposed a series of ten tests. He claimed that this standardised testing procedure could be used to determine the structure of intelligence, although the term intelligence appears nowhere in his article. 

A few years later, C. G. Spearman (1904) published a paper that described his study of Galton's hypothesis that a general mental ability underlies every kind of mental effort. He studied this uniformity of function (also known as general mental ability) using factor analysis, a statistical procedure that he had invented. He studied the relationship between different types of intelligence and student performance on both physical and mental tests. Spearman found four types of intelligence:

1. Present efficiency: the performance on (school) tests. 
2. Native capacity: derived by determining the difference between each student's rank in school and rank in age.
3. The general impression the student presented upon others; teacher's ranking as to whether the student was bright, average, or dull.
4. Common sense: sharpness and common sense out of school.

Spearman arrived at the following conclusion: "all branches of intellectual activity have in common one fundamental function (called g) whereas the remaining or specific elements (called s) of the activity seem in every case to be wholly different from that in all the others" (Spearman, 1904, p.284). This function g could be one function, or a group of functions. This paper is still of influence today and has been cited 1,327 in 2016 on APA PsychNET Gold. 

During the early twentieth century, the development of intelligence tests developed rapidly. One such test was developed by Binet and Simon (1905); they developed an individual intelligence test to help identify cognitively impaired and abnormal school children in France. Intelligence tests were now used by, among others, the American army to recruit soldiers as well as by the industry to assess the suitability of workers for specific jobs and to study presumed differences in intelligence between the sexes and races. In other words, intelligence testing during this period was driven not only by practical concerns, but also by those seeking guidance on how to address various pressing social issues. However, most of the intelligence tests used to assess both individual and group intelligence were not based on an explicit, well-defined, validated theory of intelligence, even though there was no shortage of such theories. 

What were the major attacks on Spearman's g factor? 

After Spearman's publication, it remained silent for a while. It was not until about the 1930s that major attacks on his work began to emerge.  

The first major attack to Spearman's findings came from Thurnstone (1931) who proposed multiple factor analysis to study the structure on intelligence. Thurnstone found that intelligence was not comprised of one factor, but instead of seven: visual imagery, perceptual speed, memory, word fluency, number facility, induction, and verbal relations.

A second attack on Spearman's general intelligence factor came from one of his students; Guilford (1967). Guilford proposed a three dimensional classification model of intelligence.

Another attack on Spearman's work came from R.B. Catell (1971), who began publishing doubts from the 1930s onwards. After several publications, Catell published a book in 1971 entitled Abilities, their structure, growth, and action. In this book, he summarised his prior papers and spent a considerable amount of time discussing the rationale of gf (fluid intelligence) and gc (crystallised intelligence).

Fluid intelligence (fc)

Fluid intelligence (gf) appears to operate whenever the sheer perception of complex relations is involved. It is an expression of the level of complexity of relationships which an individual can perceive and act upon when he does not have resources to answers to complex issues already stored in memory. Fluid intelligence is largely hereditary. 

In other words, fluid intelligence is considered an innate, neurologically based phenomenon. It is, in essence, a novel problem-solving capacity that demonstrates itself when crystallised intelligence abilities are of little or no use. Fluid intelligence determines the upper limit of the amount of crystallised intelligence one accumulates. Fluid intelligence declines with age as a consequence of the neurological insults that occur from adverse environmental events and normal aging.

Crystallised intelligence (gc)

Crystallised intelligence (gc) is the set of judgemental skills from investigating gf in those cultural activities which need high gf. The form of crystallized intelligence is never the same in two cultures. Crystallised intelligence arises not only from better educational opportunity, but also from a history of persistence and good motivation in applying fluid intelligence to approved areas of learning. In contrast to fluid intelligence, crystallised intelligence results from learning and is more influenced by the amount of education and learning experiences to which an individual is exposed. It may take the form of implicit or explicit semantic knowledge, and skills. 

Nowadays, research is preceding with ongoing research about G and GF-GC, offering refinements of these theories. Overall, it can be stated that the gf-gc theory has been accepted into the mainstream. Evidence therefor comes from test publishers like NCS Pearson, who have explicitly incorporated the gf-gc theory in their test development. 

In summary, it appears that although the concepts of g, gf, and gc have strong empirical support, there is an ongoing debate in regard to whether g is comprised of one, two, or more factors. 

What is currently known about the neuropsychology and neurology of GF?

The final section of this paper briefly addresses some of the putative neuropsychological components of gf and the neurological substrates underlying this domain. As noted earlier, fluid intelligence is defined as a general, relation-perceiving span, or the capacity to perceive relations. In order to perceive such relations between two or more objects, one has to be able to hold those two objects in mind and access one's semantic memory system with regard to what is known about each in order to ascertain their similarities and differences. To do so, working memory is essential. In fact, Kyllonen and Christal (1990) have suggested that working memory capacity might be the neuropsychological mechanism that is responsible for gf. If working memory and fluid intelligence are strongly related, it makes sense that if one could improve the working memory of an individual through training, it should yield a positive effect on the fluid intelligence of that individual. This was, however, not found to be the case in multiple studies. For example, Gignac and Watkins (2015) found that the association between working memory capacity and gf was insignificant after controlling for the general effects of general intelligence. Hence, it appears that the precise relationship between working memory, attention, and fluid intelligence, has yet to be determined. For now, the precise neuropsychological and neurological components of gf remain unclear. It is clear, however, that fluid intelligence involves components which include (among others), short-term memory, working memory, long term memory, and various attention and executive functions. 

In summary, the present paper briefly discussed the nature, origin, and evolution of Catell's (194) gc-gf theory. In doing is, it discussed the seminal work of Spearman (1904) with his discovery of a general function of intelligence, called g, as well as the challenges to his finding. The paper ended with some of the possible neurological substrates of gf.