Understanding IQ Calculation and Mental Age in Modern Psychological Assessments

IQ has transformed significantly since the early 20th century. Initially, the ratio method for calculating IQ was devised by William Stern. However, this method was replaced by the deviation method introduced by David Wechsler. The current definition of IQ is based on the deviation method, where IQ is calculated as 15 times the z-score plus 100. The z-score is relative to the age group, ensuring that the score reflects a standard deviation within a particular age cohort. Nonetheless, the concept of mental age remains relevant in certain psychological assessments, particularly for children under the age of 16.

The Evolution of IQ Calculation: From Stern to Wechsler

William Stern, a prominent German psychologist, introduced the ratio method of calculating IQ in the early 1900s. The method he proposed used the formula: Mental Age (MA) over Chronological Age (CA) times 100. This approach was straightforward but had limitations, leading to its eventual abandonment. In the late 1930s, David Wechsler, another influential psychologist, introduced the deviation method, which has since become the standard in modern psychological testing.

The Deviation Method and Z-Score

The current definition of IQ is based on the deviation method, where IQ 15 × z-score 100. The z-score is a statistical measure of the disparity between a raw score and the mean score for a given age group, adjusted for standard deviation. For instance, a z-score of 1 indicates that the raw score is one standard deviation above the mean.

Mental Age: Still Relevant for Younger Individuals

Although the concept of mental age is no longer central to standard IQ calculation, it is sometimes used to assess younger individuals, particularly those under 16 years of age. To calculate mental age using mental age IQ × CA, one can determine the mental age by multiplying the IQ score by the chronological age. For example, an IQ of 95 for a child aged 10.5 years would result in a mental age of approximately 9.975 years, or nearly 10 years.

The Genetics of Intelligence: Polygenic Scores and Genome-Wide Association Studies (GWAS)

Intelligence is a complex trait influenced by both genetics and the environment. Recent studies, such as those by Sophie von Stumm and Robert Plomin, have shown that polygenic scores can be derived from genome-wide association studies (GWAS) to predict an individual's level of intelligence. These scores can be calculated for embryos, newborns, and adults, providing new insights into the genetic basis of intelligence.

Genome-Wide Association Studies (GWAS) and Polygenic Scores

Robert Plomin's book, Blueprint: How DNA Makes Us Who We Are, explores the relationship between genetics and intelligence. GWAS and polygenic scores are powerful tools in understanding the genetic determinants of intelligence. These methods can help predict an individual's predisposition to certain traits, including intelligence, even from an early developmental stage.

Conclusion

The evolution of IQ calculation has seen significant changes from the ratio method to the deviation method, with the latter being the current standard. While mental age is not directly calculated in modern IQ tests, it remains relevant for younger individuals. The use of genetic studies, such as GWAS and polygenic scores, provides a new framework for understanding the genetic basis of intelligence, offering insights into the genetic and environmental factors that influence this complex trait.