Sep 10, 2025·Advanced Topics & Research

Cognitive Development

Explore how IQ scores evolved from Binet’s mental age and quotient IQ to modern deviation IQ, showing child development, adult comparisons, and age-related changes.

Dr. Russell T. WarneChief Scientist

One of the insights that the first IQ test creator, Alfred Binet, was that children’s minds develop as they get older. An implication of that insight was that the cognitive abilities of smarter children would resemble those of children older than them. (Likewise, Binet believed that the cognitive abilities of less intelligent children would resemble the abilities of children like them.) Binet was correct in his beliefs about cognitive development during childhood, as many parents can confirm.

Cognitive Development and IQ

Binet’s insight into how children can be compared to other children at different ages led to the concept of “mental age,” where a child’s test performance would be expressed in terms of the average age of children who performed equally well. Originally, IQs were calculated by dividing the mental age by the child’s actual (i.e., chronological age) and multiplying by 100. Today, the formula that produces this IQ is called a “quotient IQ.” For quotient IQs, an IQ of 100 is average at any age, and children with scores higher than 100 performed as well on the test as older children. On the other hand, children whose scores are less than 100 perform as well as children who were younger than them.

The Deviation IQ

Binet only studied cognitive development in childhood, but it is clear that the cognitive development doesn’t continue in adulthood. If it did, then a 40-year-old would be as noticeably smarter than a 35-year-old as a 10-year-old and a 5-year-old. For this reason, the quotient IQs never fully made sense for adults. (The problem was temporarily “solved” by setting every adult’s chronological age to 16 for calculation purposes, but this was never very satisfactory.) This is one of the reasons why test creators changed to a “deviation IQ.” In this method, examinees are compared to one another, and the number of standard deviations above or below the average is calculated for each examinee. In a normal distribution (pictured below), 68% of people have a score within +1 standard deviation, and 95% have a score within +2 standard deviations. These scores are then converted to IQs that have an average of 100, with each standard deviation being a 15-point span.

Deviation IQs are more complicated to calculate, but they make sense for calculating IQs for adults. They also solve other problems from quotient IQs. For example, quotient IQs could not be compared across ages. An IQ of 120 did not mean the same thing at age 5 and age 12. Because of the problems with quotient IQs, every IQ test has used deviation IQs in its score calculations. But the term “IQ,” which originally stood for “intelligence quotient” stuck around.

One important fact to note with deviation IQs is that they make relative comparisons, not absolute ones. In other words, they state how well an examinee performed on a test compared to other people in their age group. That is very valuable information, but it ignores the year-to-year growth that occurs throughout childhood. With deviation scores, a child who performs as well as the average child in their age group receives a score of 100, even though a 16-year-old child with an IQ of 100 has much more impressive cognitive abilities than an 8-year-old with an IQ. Absolute comparisons like these require other scores and calculations.

Cognitive Development in Adulthood

Although cognitive abilities do not grow from year to year in adults, that does not mean development stops. Adults still learn new information, especially if they attend college or expose themselves to information by reading or engaging in educational activities. In late adulthood, What does occur in adulthood, though, is some decline in late adulthood. Even when dementia is not present, older adults experience noticeable declines -- especially in processing speed, working memory, and fluid reasoning. Still, it is not all bad news for seniors. Learned knowledge, verbal abilities, and certain memory tasks do not decline until very late in life (the mid-to late 80s, on average). Using a deviation IQ to compare adults to those in their age group controls for these changes that occur in adulthood.