What is the 'reverse Flynn effect,' where people's intelligence gradually declines?

In the 20th century, the ' Flynn effect ' was observed, in which average IQ test scores increased with each generation. However, in recent years, the opposite, the ' reverse Flynn effect ,' where scores decline, has been reported in some countries. A study of approximately 394,000 adults in the United States also confirmed a decline in scores on multiple cognitive ability tests between 2006 and 2018.

Looking for Flynn effects in a recent online US adult sample: Examining shifts within the SAPA Project - ScienceDirect
https://www.sciencedirect.com/science/article/pii/S0160289623000156

Are People Actually Getting Dumber? Introducing The Reverse Flynn Effect : ScienceAlert
https://www.sciencealert.com/are-americans-actually-getting-dumber-heres-what-we-know-about-the-reverse-flynn-effect

In the 20th century, a phenomenon was observed in various countries where the average score on IQ tests increased with each subsequent generation. This phenomenon is called the 'Flynn effect,' named after James Flynn, who published his research findings in 1984, and it is estimated that the average score increased by approximately 3 to 5 points every decade.

Various hypotheses have been proposed to explain the Flynn effect, including improved nutrition, a lower age of starting education, a decrease in the number of siblings, and the increasing complexity of daily life and work environments. However, in recent years, cases have been reported in Finland and France where the IQ test scores of the new generation are lower than before, and the 'reverse Flynn effect,' which shows a change in the opposite direction to the conventional one, has attracted attention.

Elizabeth Dwolak and colleagues at Northwestern University analyzed data from the Synthetic Aperture Personality Assessment (SAPA) Project, an online survey project, to investigate whether the reverse Flynn effect occurs in American adults. The study included 394,378 participants aged 18 to 90 who grew up in the United States, and the responses were collected from April 2006 to December 2018.

Participants answered questions from the open-source cognitive ability test 'International Cognitive Ability Resource (ICAR).' In addition to the overall score, the research team individually analyzed four areas: 'matrix reasoning' (identifying patterns in shapes), 'letter and number sequence reasoning' (predicting the next element from a sequence of letters or numbers), 'verbal reasoning' (using vocabulary and general knowledge), and '3D rotation' (rotating three-dimensional objects in one's mind).

The analysis revealed a downward trend in overall scores on the 35-question test from 2006 to 2018. This trend was confirmed even when analyzed separately by age, education level, and gender, and a pattern of lower average scores was observed in matrix reasoning and letter/number sequence reasoning among those who participated in the latter half of the survey period.

The decline was particularly large among participants aged 18 to 22. Furthermore, while the inverse Flynn effect was observed across all educational levels, the slope of the score decline was relatively steeper among those without a four-year university degree, indicating differences in the degree of change depending on educational level.

However, not all abilities declined. While the changes in verbal reasoning did not reach the criteria the research team considered statistically significant, scores in 3D rotation increased by an average of 0.030 standard deviations per year from 2011, when measurements began, to 2018, confirming the traditional Flynn effect in many age groups.

These results do not mean that 'modern people have simply become dumber.' Changes in IQ and cognitive test scores do not necessarily reflect a fundamental increase or decrease in human intelligence; they may be influenced by factors such as familiarity with the test, educational environment, culture, and the information technology used. The research team itself states that they could not pinpoint the cause of the score changes from this study.

Furthermore, the SAPA Project data is not from a randomly selected, nationwide representative sample, but from people who found and participated in the online survey themselves. Because the demographic composition is skewed, with over 65% of participants being women, caution is needed when generalizing the results to the entire United States.

The research team points out that the inverse Flynn effect may not be a phenomenon that occurs uniformly regardless of age or education level. These results suggest that cognitive abilities as a whole are not changing in one direction, but rather that the composition of abilities may be changing in response to modern culture and living environments. For example, scores may decrease in abstract and logical reasoning while scores may increase in spatial reasoning.