Some research topics are polarizing. It, of course, depends on the time and on the country. One prominent example is the topic of human intelligence. Intelligence research was and is socially and politically charged. IQ (Intelligence Quotient) tests had been used in the past to justify discrimination, eugenics, and racial hierarchies. Nowadays, findings about group averages may be misinterpreted as moral judgments for or against certain groups. However, it doesn’t have to be like that. IQ tests are just another tool to understand human cognition and brain function. This time, I read a book titled “IQ and Human Intelligence” by Nicholas Mackintosh.

The first IQ tests in the early 20th century were developed to assess the educational abilities of children and to identify those who needed support. Later, during World War I, the US military adapted the tests to measure the abilities of recruits with the goal of assigning them to a particular role. Over time, the usage of IQ tests has become more widespread. Now, intelligence assessment is applied in education (to identify gifted children and those who need support), in clinics (to diagnose cognitive impairments), and in cognitive science research (to study interindividual differences and their underlying neuronal substrate).

In a typical IQ test, an individual will face the following problems:

1) Verbal reasoning—the ability to build analogies, for example, 

Book is to Reading as Fork is to:

a) Drawing
b) Writing
c) Eating
d) Cooking

Answer: c) Eating

2) Numerical reasoning—the ability to operate with numbers, for example,

Insert correct signs into the equation:

5 … 6 … 2 = 28

a) + -
b) - -
c) / -
d) * -

Answer: d)

3) Spatial reasoning—the ability to see patterns in figures and complete patterns, for example,

Which figure is the correct one in the row?

➡️ ⬇️ ⬅️ ⬆️ ➡️ ⬇️ ___ (What comes next?)

Options:

a) ⬅️
b) ⬆️
c) ➡️
d) ⬇️ 

Answer: a)

In the early days of IQ testing, a question arose whether those tests assessed different independent abilities or one single factor. It could be that those tests measured different facets of human intelligence with little or no overlap. It also could be that tests measured not only specific abilities but also, to a certain extent, some underlying factor common to all tests (~intelligence). This question still has not been answered.

Nonetheless, it turned out to be useful to distinguish abilities. A common distinction currently used in the research is between fluid (Gf) and crystallized (Gc) intelligence. Gf is the ability to reason abstractly and solve new problems without relying on pre-existing knowledge or experience. Gc is the accumulated knowledge of facts and skills that were acquired through experience and education.

Implicitly, IQ tests tap into other cognitive skills that are considered distinct on their own. One of them is mental speed. The faster a person thinks, the more problems they will solve in the test. Mental speed (or also called the speed of processing) can itself be subdivided into 1) rapid sensory analysis, which is measured by inspection time (how long does a participant need to grasp the task), and 2) rapid decision-making, which is measured by reaction time (how long does it take to press the button, like in a Whac-A-Mole game). Turns out that both inspection time and reaction time contribute to IQ score.

Another cognitive skill implicated is working memory. It is the skill of maintaining several items in short-term memory and manipulating them according to the requirements of the task. A typical working memory task is the n-back task. Participants are shown a sequence of letters and must determine if the current letter matches the one presented "n" positions earlier. If the sequence is "A, B, A, C" in a 1-back task, one should respond "yes" when the second "A" appears, because it matches the first letter. It’s easy to see how this ability relates to many tasks in IQ tests. Yet, working memory is considered a separate cognitive skill.

However, even if diligent researchers control for speed of processing and working memory in their research and try to measure "real" intelligence, they still may face another problem. If, for instance, spatial ability is not someone’s forte, a person may convert spatial problems into verbal or logical ones. The person may narrate their actions: “First, I turn the cube 90 degrees; now there are two dots on the top…” Or they may use logic to eliminate incoherent options: “Option A shows the extra dot on the top. That's wrong. Eliminate A.” IQ tests are usually administered in groups, and there is little chance to know exactly how different people solve the task.

Despite problems in tests and the identification of constructs that they measure, IQ assessment is still widespread. Partly, it is due to the fact that IQ scores do correlate with many things. IQ is highly heritable. IQ is a fair predictor of job placement and actually a better predictor than biographical data, educational level, college grades, or interviews. IQ correlates with success in life, and in particular, educational success, and with criminal behavior. IQ relates to health status and longevity.

Differences in IQ between groups are attributed to genetic and environmental factors, and genes and environment interact. There are robust differences between males and females in abilities that constitute general intelligence. Males are superior to females in mental arithmetic and spatial reasoning. While females are superior to males in verbal tests, immediate memory, and perceptual speed. Males’ superiority in math may be partially explained by environmental factors. Society (including parents of a male child) believes that males are superior in math and encourages them to pursue professions involving math. Male superiority in the spatial domain may be explained by evolution. Males in ancient societies were hunters and had to traverse larger spaces than females. Interestingly, there is evidence that spatial skills may be affected by hormones; for example, transgender people who take testosterone during transition report improvement in their spatial abilities.

As I said in the beginning, for me, an IQ test is just another tool to understand the functioning of the brain. Moral, ethical, or identity statements regarding intelligence are often far-fetched and unjustified. Some people tend to bridge IQ (being clever) and the general worth of a person (or their happiness), even to the point of considering them one and the same thing. Of course, they are not the same. Moreover, the group differences are simply what they are—group differences, not precise predictions for individual life trajectories.

The book increased my curiosity about studying human intelligence. I recommend it as a starter for those who are interested in the topic.

Favourite quote:

“The average IQ of convicted criminals was some 8-10 points below the population mean. A low IQ predicts getting into trouble with the police independently of family background. One popular explanation: intelligent criminals do not get caught. A more prosaic possibility is that IQ is associated with failure, particularly at school. If you do badly at school, you are more likely to skip school, leave as soon as possible without educational qualifications, and find it difficult to obtain a job. Petty crime may well seem to offer a good solution to all these problems, and petty crime leads to more serious crime.”

November, 2025