The question of whether humanity’s collective intellect is diminishing is driven by decades of standardized testing data. For much of the 20th century, observed cognitive performance across populations showed a steady, encouraging rise. Recent findings, however, suggest that this long-term progression has stalled and, in many developed nations, has begun to reverse. Analyzing these trends requires understanding how scientists quantify intelligence and what the data reveal about performance changes over time.
How Cognitive Ability is Measured
Researchers rely on standardized tests to quantify cognitive ability across large populations. These assessments typically separate general intelligence into two primary components: fluid and crystallized intelligence. Fluid intelligence represents the capacity to solve novel problems, reason abstractly, and identify patterns without relying on accumulated knowledge. This ability is often measured using non-verbal assessments like Raven’s Progressive Matrices.
Crystallized intelligence, conversely, is the knowledge and skills acquired through learning, education, and life experience. Vocabulary, general information, and arithmetic skills are examples of this accumulated knowledge. Comprehensive test batteries, such as the Wechsler Intelligence Scales (WISC), include subtests for both types of cognitive function. These tests provide a consistent method for tracking population-level changes, but they measure performance relative to a defined norm.
The distinction between these two types of intelligence is important because they develop differently throughout a person’s life. Fluid reasoning tends to peak in early adulthood before gradually declining, while crystallized knowledge can continue to increase well into later life. This difference allows researchers to pinpoint which specific mental skills are changing when they observe a shift in overall test results. By continuously re-norming these tests, scientists track generational differences in cognitive performance.
The Historical Trend of Rising Scores
For approximately 100 years, beginning around the early 1900s, standardized test scores demonstrated a remarkable and sustained upward trajectory worldwide. This phenomenon, known as the Flynn Effect, showed that each successive generation performed significantly better than the one before it. The average gain observed across many developed countries was substantial, often approximating 3 points per decade.
This consistent increase was observed across various measures and age groups. The magnitude of the cognitive leap was such that a person from the early 20th century would likely score far below the current average on a modern standardized test. The gains were not uniform across all cognitive domains; some evidence suggests that gains in fluid intelligence, particularly abstract problem-solving, were initially more pronounced than gains in crystallized knowledge.
This historical data established a clear expectation of generational cognitive improvement. The continuous rise in average performance provided a baseline against which all subsequent test results would be compared.
Evidence for Recent Declines
The decades-long trend of rising scores began to falter in the late 20th century, leading to what is now termed the “Anti-Flynn Effect.” Studies analyzing large, mandatory testing cohorts in several developed nations have documented a clear reversal of the historic gains. In Norway, an analysis of the cognitive scores of conscripted men born between 1962 and 1991 found that scores peaked for those born in the mid-1970s and then began a steady decline. This reversal was equivalent to a loss of approximately 7 points per generation.
Similar patterns of decline have been observed in other European nations, including Denmark, Finland, France, the Netherlands, and Britain. Data from Danish male conscripts showed performance peaked in the late 1990s before moderately declining to levels seen a decade earlier. The pattern is not limited to Europe; research in the United States, utilizing data from 2006 to 2018, also revealed a decline in scores across multiple cognitive categories.
The decline appears to be most pronounced in specific cognitive domains, particularly those related to fluid intelligence and abstract reasoning. Studies in the US sample found drops in scores for verbal reasoning, matrix reasoning, and number series tasks, which are core elements of fluid ability. This suggests that the skills related to logical thinking and problem-solving are showing the sharpest deterioration.
The use of large, representative samples, such as those drawn from military conscription data, strengthens the validity of these findings. The observed decline is not attributed to a testing anomaly but rather a real change in performance within families. This indicates that environmental factors, rather than genetic ones, are the likely cause of the reversal. The evidence points to a new era where the cognitive environment no longer supports the generational gains seen in the past.
Proposed Environmental and Societal Factors
Researchers are actively exploring several environmental and societal factors that may be contributing to the observed decline in cognitive performance. One major area of investigation is the impact of neurotoxins, which can interfere with brain development and function. Exposure to heavy metals like lead and certain organophosphate pesticides has been definitively linked to measurable reductions in cognitive scores. A persistent presence of these chemicals in the environment may still be a factor, despite overall exposure reductions in developed countries.
Nutritional changes are another prominent hypothesis for the reversal of the historical trend. While improved nutrition likely drove 20th-century gains, modern diets may be failing to provide adequate levels of certain brain-essential nutrients. Deficiencies in iodine, iron, zinc, and omega-3 fatty acids are a concern in some populations, as they are important for optimal cognitive development. The increasing consumption of ultra-processed foods may exacerbate these nutritional deficits.
Changes in educational structure and media consumption also represent potential mechanisms for the decline. Some researchers suggest that a shift in educational focus away from abstract problem-solving and toward rote learning may reduce the development of fluid intelligence. The pervasive integration of digital technology and social media may also be altering the way people process information. Increased reliance on passive, digital tools might reduce the cognitive load required for daily tasks, thereby diminishing opportunities to exercise abstract reasoning and systematic planning skills.