To open the discussion, let's talk about IQ itself. Like it or not, the concept of IQ is still with us. How often do you read a book and the IQ of a character is referred to? Or you turn on the news and find out that a defendant's low IQ is being used as mitigating circumstances for a crime? Indeed, while they may be given different names like "cognitive-ability tests" or "aptitude tests", IQ tests are still frequently used in both school and work assessments.
But while most people know that having a higher IQ is more desired, the concept of IQ has led to much confusion over just what intelligence is. For instance, most people know that IQ stands for "Intelligence Quotient". But why is it called a quotient? It is also claimed that a person's IQ is relatively stable over the lifespan. But does this really make sense? Is a child not smarter at sixteen years of age compared to when they were just five? It turns out that intelligence does increase over childhood, even though IQ hides this improvement. To understand this, we need to look at how IQ tests were designed to measure intelligence.
A person's IQ score was originally calculated by dividing a person's mental age by their chronological age, and then multiplying by 100. In other words, MA ÷ CA × 100. Chronological age was simply the child's biological age, or how old the child was. Mental age was given by first giving the child an intelligence test and seeing how they scored. This score was then compared with how other children did on the test. It was found that as children get older, their score on the test would improve. Mental Age was then defined as the age at which a typical child exhibits a particular level of performance on the test. In other words, how old is the average child when they score that high on the test? So IQ is a measure of the mismatch between a child's Mental and Chronological Ages.
Let us look at a couple of examples to illustrate this. Imagine if a twelve-year-old performs like typical twelve-year-olds on the test. Their IQ will then be calculated as 12 ÷ 12 x 100 = 100. An IQ of 100 represents the average IQ score, because the child's Mental and Chronological Ages match. But what if a child is gifted for their age? This means that they perform better than a typical child of the same age. They might perform like a typical child who is two years older. Hence, the twelve-year-old child will then have a Mental Age of a fourteen-year-old. Their IQ will then be 14 ÷ 12 x 100 = 116. So an IQ score greater than 100 indicates that a child is performing better than the average child of that age on the test.
We can then see that the Intelligence Quotient was called a quotient as it involved dividing by Chronological Age. This indicates that while IQ stays stable over childhood, this does not mean that actual intelligence stays the same over this time. To the contrary, IQ is only able to stay stable over childhood because Mental Age, or the ability to solve intelligence problems, is steadily increasing. If Mental Age stayed the same over childhood, then a child's IQ would steadily decrease as they get older.
This increase in Mental Age is reflected in the design of IQ tests. The problems that are used to identify gifted five-year-olds are quite easy, such that any adult would find them easy to answer. On the other hand, the problems that are used to identify gifted sixteen-year-olds are so difficult that many adults will struggle to solve them.
More recent IQ tests use a procedure whereby a person's performance is directly compared to the performance of other people of the same age, often referred to as a normative sample. However, despite this difference in procedure, the underlying variations in intellectual performance are the same.
The essential point to take away here is that IQ being stable does not indicate that mental or intellectual performance is stable. IQ is only stable because of how IQ represents differences in intelligence across people - it is not a direct measure of how people perform on an intelligence test, but a comparison with other people of the same age. Appreciating this characteristic is important when we start to look at what it is in the brain that IQ tests may be assessing.