History of Intelligence Theories

Charles Spearman Reading Recommendations

Selected publications Comments
“General intelligence, objectively determined and measured” (Spearman, 1904) The work that started it all. Along with the historical review, statistical analysis, and some raw data, here and there, you get delicious bits of rhetoric like this:

But when we assert that the decision of Regulus to vote against making peace with Carthage was no more than a conglomeration of visual, auditory, and tactual sensations in various stages of intensity and association, then there is an undeniable risk that some precious psychical elements may have slipped through our fingers. (p. 204)

The nature of “intelligence” and the principles of cognition (Spearman, 1923) Spearman considered this book to be his most important work (Jensen, 1994). The book is easier to appreciate if you think of it as the work of Spearman the philosopher—to whom we grant the privilege of asserting things without really explaining or justifying those assertions. The ideas themselves are fascinating. The empirical justification for them mostly comes in later works.
The abilities of man: Their nature and measurement (Spearman, 1927) Most scholars consider this to be his most important work (Jensen, 1994). Although the Spearman-Brown prophecy formula holds a special place in my heart, I must agree.
C. Spearman (Spearman, 1930) Reading Spearman’s autobiography makes it hard to dislike the man. Funny, moving, and insightful.
Cognitive Assessment

Cattell’s thoughts about Spearman

Raymond Cattell studied with Charles Spearman. From Cattell’s (1974) autobiography, we can see that Spearman’s gratitude toward his mentor Wundt was repaid with the same level of gratitude and respect from Cattell:

Great men intrigue me, with something of the feeling one gets before a lofty mountain or other magnificent nature a spectacle but with more significance. Despite a marked shyness as a young man, I have been privileged to know rather well several men in psychology and associated areas I would call great or near-great—Spearman, Sir Cyril Burt, Haldane, Sir Ronald Fisher, Sir Godfrey Thomson, Thurstone, Thorndike, Terman, Lashley….It is not my purpose to try any ranking among them—their virtues are too diverse—but Spearman was uniquely great. He conveyed on the one hand, a sense of historical depth of scholarship, and on the other, an almost naïve freshness of approach. He would take up a conversation as if he had just been talking to Plato, and then stagger you with sharp experimental novelty that no contemporary psychologist had thought of. For this unworldly and absent-minded man, life was pared down to essentials: the fascinated pursuit of truth, the encountering of life’s absurdities by a sanity of droll humor, and an affection for family and coworkers. (p. 65)

Cattell (1987) also relates this fun story about his mentor giving a talk about the importance of specific abilities:

An anecdote of that meeting may be of interest as illustrating the absent-mindedness of a great theorist. Before a large audience Spearman reeled off, without writing on the chalk board, the various complex formulae supporting his main theoretical position. Perceiving the expressions of the audiences, the present writer, as research assistant to Spearman, ventured to put a piece of chalk in his hand. He held it faithfully to the end of the hour and then, saying, “And this is what I call the theory of ‘g’,” he wrote one small and very solitary “g” in the middle of the large board! (p. 31)

Cognitive Assessment

“Strawman Spearman” vs. Charles Spearman

Many psychologists have strong negative opinions about Charles Spearman’s ideas about intelligence. Unfortunately, those opinions are often based on distorted secondary sources. A straw man version of Spearman is easy to dismiss. The real Spearman is a far more nuanced thinker and formidable foe. There is much with which one can disagree in Spearman’s writings, but it is generally preferable to disagree with what he actually believed than with what others say he believed.

“Strawman Spearman” Charles Spearman
  • Intelligence is unidimensional.
  • Intelligence is multidimensional (Spearman, 1927, pp. 222–241).
  • General intelligence is the only aspect of intelligence that matters.
  • In many situations, specific abilities matter quite a bit, sometimes much more than the g factor (Spearman, 1925):

Since the very beginning it has been known that the two factors, g and s, the energy and the engines, may have widely different relative importance, according to the particular mental operation involved. With some operations the superiority of one person over another is preponderantly decided by their respective amounts of the energy. With other operations, on the contrary, the dominant factor is the engine.

Subsequent research, moreover, has been gradually outlining the cases which incline in the one or the other direction. Thus the energy is in general more important for operations that are composite, the engines for those that are monotonous. This is natural enough. The composite operation really involves several different engines; the superiority that an individual may happen to have in any one of them will tend to be neutralised in the average of them all; but a superiority in the energy will make itself felt in each, and thus obtain cumulative influence. (pp. 178–179)

  • There are specific factors of ability and there is g. There are no group factors of intelligence.
  • Actually, there are many group factors but only a few of them are broad and important (Spearman, 1927):

We have now arrived at the “group factors” which have played such a baffling part in controversial writings. They make their appearance here, there, everywhere, nowhere; the very Puck of psychology. On all sides contentiously advocated, hardly one of them has received so much as a description, far less any serious investigation.

And yet they are of immense importance, not only theoretically, but also practically. By dint of nothing else can all those who claim to measure “special abilities” holding out magnificent promises for industry—be saved from the charge of living in the fool’s paradise of “faculties.” For a test only measures any ability other than g by having correlation with it other than that due to g. (pp. 222–223)

Note: I am guilty of repeating the false fact that Spearman was reluctantly persuaded by Thurstone that group factors exist (Schneider, 2013). As seen in this quote, Spearman had found evidence for group factors more than a decade before Thurstone’s major publications.

  • The g factor is a reified thing, an ability.
  • The g factor is an empirical regularity that demands an explanation. It is not an ability but an influence on all abilities (Spearman, 1927):

…notice must be taken that this general factor g, like all measurements anywhere, is primarily not any concrete thing but only a value or magnitude. Further, that which this magnitude measures has not been defined by declaring what it is like, but only by pointing out where it can be found.

[…]Eventually, we may or may not find reason to conclude that g measures something that can appropriately be called “intelligence.” Such a conclusion, however, would still never be the definition of g, but only a “statement about” it. (pp. 75–76)

  • g is awesome.
    IQ tests measure g.
    ∴ IQ tests are awesome!
  • Spearman regularly criticized the “hotchpotch” nature of IQ tests. Spearman (1931):

All in all, then, the plea is here raised that the current procedure of testing “intelligence” needs to be aroused from its self-complacent slumber. The present real basis of it is only a practice that has been borrowed from another theory. And this practice fails to be satisfactory because, when the practice was borrowed, the theory itself was left behind. By so doing, a sacrifice was made of precision; of meaningfulness; and above all, of an immense amount of observed fundamental facts. This disastrous situation has been largely masked, but at the same time really aggravated, by usurping the pretentious and
wellnigh fraudulent title of “intelligence.”
(p. 410)

  • If you don’t have a lot of g, you are pretty much doomed to a lifetime of dreadful monotony.
  • Not so! Spearman (1925):

Every normal man, woman, and child is, then, a genius at something as well as an idiot at something. It remains to discover what—at any rate in respect of the genius.This must be a most difficult matter, owing to the very fact that it occurs in only a minute proportion out of all possible abilities. It certainly cannot be detected by any of the testing procedures at present in current usage. But these procedures are capable, I believe, of vast improvement. The preceding considerations have often appealed to me on looking at a procession of the Unemployed, and hearing someone whisper that they are mostly the Unemployable. That they are so actually I cannot help concurring. But need they be so necessarily? Remember that every one of these, too, is a genius at something—if we could only discover what. I cherish no illusion, indeed, that among them may be marching some ‘mute inglorious Milton, some Cromwell guiltless of his country’s blood.’ For these are walks in life that appear to involve a large amount of g. But I am quite confident that every one of them could do something that would make him a treasure in some great industrial concern. (p. 181)


Schneider, W. J. (2013). What if we took our models seriously? Estimating latent scores in individuals. Journal of Psychoeducational Assessment, 31, 186–201.

Spearman, C. E. (1925). Some issues in the theory of “g” (including the law of diminishing returns). Nature, 116, 436–439.

Spearman, C. E. (1927). The abilities of man: Their nature and measurement. London: Macmillan.

Spearman, C. E. (1931). Our need of some science in place of the word “intelligence.” Journal of Educational Psychology, 22, 401–410.