Have you ever wondered why IQ scores always cluster around 100? At the origin of this number lies an idea that has largely been retired today, yet remains deeply intuitive: mental age. Although modern tests now use a different method, understanding mental age is the easiest way to grasp what IQ actually means. Because it all started with a simple comparison.
What Is Mental Age?
The idea of mental age rests on expressing a child's mental performance by comparing it to the average performance of a specific age group. Suppose an 8-year-old child solves questions typically solved by 10-year-olds. That child's "mental age" is considered 10, even though their chronological age is 8. The idea is simple and clear: turning performance into an age.
How Did the Formula Work?
Early IQ calculations divided these two numbers and multiplied by 100: mental age divided by chronological age, times one hundred. An 8-year-old with a mental age of 10 would thus have an IQ of 10 divided by 8 times 100, which equals 125. If mental age equals chronological age, the result is exactly 100. This is the origin of the statement "average IQ is 100": the scenario where your mental age equals your actual age, meaning you score right in the middle of your peers.
Why Is It Intuitive?
The beauty of this method is that it links an abstract score to something concrete: age. Saying "their IQ is 125" might not mean much at first, but saying "they are 8 years old, but think like a 10-year-old" immediately creates a clear picture in everyone's mind. That is why mental age allowed the concept of IQ to be widely understood and popularized. It turned a score into an everyday concept.
The Big Problem: Adults
However, this formula had a serious flaw. While mental development in children progresses rapidly with age, this pace stops in adulthood. A 40-year-old person does not have twice the mental age of a 20-year-old; after a certain maturity, the mind does not grow proportionally with age. If you applied the formula as it was, everyone's IQ would appear to mathematically decrease as they aged, which is obviously absurd. While the method worked for children, it collapsed for adults.
The Modern Solution: Deviation IQ
Because of this issue, contemporary tests abandoned the mental age formula. The method used today is known as "deviation IQ": a person's score is compared to their own age group and calculated based on how much it deviates from that group's average. Thus, an 8-year-old child is compared only to 8-year-olds, and a 40-year-old adult only to their own age group. The average is still kept at 100, but the unfair influence of age is completely eliminated.
Standard Deviation and Distribution
Deviation IQ places scores on a bell curve. Most people cluster near the average of 100; as you move away from the average, the number of people receiving that score drops rapidly. That is why very high or very low scores are rare. How "unusual" a score is depends on how many units it deviates from the average. This system allows fair comparisons between people of different ages.
The Lasting Legacy of a Simple Idea
Mental age is the ancestor of the IQ concept and the most intuitive way to understand why scores cluster around 100. Even though modern tests use a fairer method, this simple idea still lies at the root of the logic: comparing a person's performance to the average of people their own age. Even if methods change, the question "where do you stand compared to your peers?" always remains at the heart of IQ.
If you want to see how your own score is calculated under today's system, you can take our free IQ test; to enjoy exercising your cognitive skills, you can check out our brain games.