Is your 'mental age' a scientific fact or a historical relic?

Nature · Published 18 October 2026 Swipe more like this →

In short: For decades, we used mental age to measure how 'mature' someone was compared to their birth date. But as psychology evolved from measuring skull sizes to complex cognitive deviations, this concept became so controversial that some now view it as an inaccurate and even infantilizing way to define human potential.

In 1905, the Binet-Simon Intelligence Test was launched by French psychologist Alfred Binet, who pioneered the notion of mental age. Binet designed the test to identify children who needed extra educational support. He observed that while older children generally performed better, some younger children outperformed their peers. He assigned these children a 'mental age'—the age at which the average child would achieve that same score. Crucially, Binet believed intelligence was not a fixed trait; he argued that mental age could change based on teaching methods and life experiences.

The concept faced immediate friction in the United States. While Binet saw mental age as a fluid indicator of growth, many American psychologists in the 1910s, including Henry Herbert Goddard, viewed intelligence as a fixed, hereditary quantity. This tension eventually led to a shift in how we measure cognitive ability. By 1955, David Wechsler introduced the Wechsler Adult Intelligence Scale (WAIS), which moved away from calculating a mental age ratio. Instead, it focused on 'deviation IQ,' which compares an individual's performance to others of the same chronological age, setting a standard average of 100.

Today, the term is largely sidelined in modern testing. In 1926, psychologist L.L. Thurstone famously labeled the concept of mental age a 'failure' due to its inconsistencies. Furthermore, the term has faced modern criticism from disabled activists and researchers who argue that it is reductive and infantilizing. While recent studies do suggest a connection between mental age and biological age, the focus has shifted toward understanding intelligence as a broad, multi-faceted ability influenced by both heredity and environment, rather than a single, permanent number.

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