Scientists have found a geospatial link between soil fertility and national intelligence scores

A recent study published in Scientific Reports suggests a notable geographical link between global soil fertility and the average intelligence quotient of nations. The findings provide evidence that the nutritional quality of local soils might play an indirect role in shaping human cognitive development on a worldwide scale.

Human brain development relies heavily on adequate nutrition, particularly the intake of essential minerals and vitamins. Plants and animals absorb these nutrients from the earth, meaning human diets are deeply connected to the health of the ground beneath their feet. When soil lacks vital elements like iron, zinc, or iodine, the food grown in it tends to be nutritionally deficient. Deficiencies in these specific nutrients are known to negatively affect cognitive growth, especially in young children.

Zinc and iron are necessary for the central nervous system to build physical structures and produce the chemicals that allow brain cells to communicate. Severe or long-lasting dietary shortages can lead to persistent cognitive impairments and learning difficulties. ”

The nutritional quality of food ultimately begins with soil,” said Sabit Erşahin, a professor of soil science at Iğdır University in Turkey. “Healthy soils support healthy crops, healthy crops support good nutrition, and good nutrition supports cognitive development.”

The author explained that the hypothesis originated from his background in agricultural science. “As a soil scientist, I have long suspected that there may be a link between soil fertility and human intelligence,” Erşahin said. “More specifically, I have wondered whether the ability of soils to supply plants with essential micronutrients, and the duration over which human populations have relied on these nutrient sources, might influence cognitive development.”

“Over long periods of time, such influences could potentially contribute to a form of inherited soil-human relationship,” Erşahin noted. Testing this idea required a way to compare vast geographic datasets. “For many years, however, I struggled to determine how such a complex hypothesis could be investigated,” Erşahin said. Questions regarding study design, data acquisition, and appropriate methods for examining the relationship between intelligence levels and soil nutrient status remained difficult to answer.

“One point I would like to emphasize is that this study began with a simple observation and a scientific question rather than a predetermined conclusion,” Erşahin explained. “A turning point came when I searched for a global map of national IQ levels.” Upon comparing this map with a world soil map, the author noticed striking spatial similarities between the distribution of certain soil types and patterns of intelligence scores.

“I also noticed a resemblance between global patterns of soil pH and IQ distribution,” Erşahin added. “These observations motivated me to explore more systematically whether measurable relationships might exist between soil properties and human intelligence at a global scale.”

The researchers analyzed data from 126 countries to evaluate the spatial relationship between soil quality and average intelligence scores. They sourced this information from the World Population Review website.

To measure earth quality, they created a specific metric called the Soil Fertility Index. This metric combined global data on predominant soil categories with their median pH levels.

In agricultural science, pH is a scale used to specify the acidity or basicity of a substance. A soil pH of around 6.5, which is slightly acidic to neutral, is generally considered ideal for plant growth. When soil becomes too acidic or too alkaline, plants struggle to absorb essential nutrients, which degrades the nutritional value of the resulting crops.

The scientists scored different soil categories based on their natural ability to support agriculture. Highly fertile soils common in temperate regions, known as Mollisols, received the highest scores. These are nutrient-rich soils often found in expansive grasslands and are known for producing high-quality crops. Heavily weathered and nutrient-poor soils typically found near the equator received the lowest scores.

These tropical soils, called Oxisols, are shaped by extreme heat and heavy rainfall, which wash away essential minerals. The researchers also factored in how much the local soil pH in each country deviated from the ideal 6.5 benchmark. To understand the geographical spread of these factors, the scientists used a technique called geostatistical analysis.

This branch of statistics focuses on analyzing spatial data to understand how variables change across physical landscapes. A specific tool within this method, known as a semivariogram, helps researchers measure how much similarity exists between data points based on the physical distance separating them. By mapping the Soil Fertility Index alongside average national intelligence scores, the scientists could mathematically compare the spatial structures of both variables.

The findings revealed a significant statistical correlation between a country’s soil fertility and its average intelligence score. “The study identified a statistically significant association between soil fertility and mean national IQ,” Erşahin told PsyPost. “The study found a moderate positive correlation (r = 0.58) between its Soil Fertility Index (SFI) and national IQ, with soil fertility explaining about 34% of the variation in national IQ across the 126 countries analyzed.”

In statistics, an r value represents a correlation coefficient, which measures the strength and direction of a relationship between two variables. This metric ranges from negative one to positive one, with a score of zero indicating no connection at all. A positive number provides evidence that as one variable increases, the other tends to increase as well.

“An r value of 0.58 is generally considered a moderately strong correlation in social and environmental sciences,” Erşahin explained. “In complex human-development research, where outcomes are influenced by many interacting variables, explaining one-third of the variability is often considered noteworthy.”

“The key message stands out: Soil may matter more than we usually think,” Erşahin added. “The paper argues that soils influence the availability of micronutrients such as iron, zinc, and iodine through the food chain. These nutrients are already known to be important for brain development and cognitive function.”

According to the analysis, regions with the lowest soil fertility and lowest average intelligence scores were heavily concentrated in tropical areas. These equatorial areas are dominated by highly acidic soils that have very limited ability to retain trace metals. In these regions, elements like aluminum can become toxic to plants, further restricting agricultural productivity.

“Regions with soils that are naturally poor in nutrient availability may face greater challenges in maintaining optimal nutrition over generations,” Erşahin noted. “Environmental conditions may contribute to differences in human development alongside many other factors.” Areas like North Africa and the Middle East also showed low to moderate scores across both variables.

These specific regions are characterized by arid soils and shallow earth, known as Aridisols and Entisols. These specific earth types tend to be highly alkaline and lack organic matter, which severely restricts plant nutrition. On the other hand, countries situated in North America, Europe, and parts of Northern Asia generally featured highly fertile soils alongside higher average intelligence scores.

The statistical models indicated that the environmental processes shaping soil quality and the factors influencing cognitive development operate over similar spatial distances. “This effect size is substantial enough to suggest that soil-related environmental factors may play a meaningful role in shaping long-term human development,” Erşahin said.

“However, the majority of the variation in national IQ remained unexplained by soil fertility, indicating that numerous other factors, including education, healthcare, socioeconomic conditions, governance, culture, nutrition, and genetics, also contribute importantly to cognitive outcomes.”

The researchers emphasize the need to avoid drawing direct causal conclusions. “As I began this study, I quickly came to realize that the subject is far more complex, multifaceted, and sensitive than I had initially anticipated,” Erşahin noted. “Before undertaking the study, I did not fully appreciate the extent to which cognitive development is influenced by the interaction of environmental, nutritional, social, economic, cultural, and genetic factors.”

The study reinforced his belief that no single factor can adequately explain such a complex human characteristic. “If I were to identify the single most important misinterpretation to preempt, it would be: ‘Correlation is not causation,’” Erşahin explained. The paper demonstrates a spatial association, but it does not demonstrate that soil fertility causes differences in intelligence.

“The study should be viewed as generating a new hypothesis about environmental influences on human development rather than providing definitive evidence of a causal mechanism,” he added. “This point is particularly important because the topic touches on intelligence, a subject that is scientifically, socially, and ethically sensitive.”

The authors point out a statistical trap known as the ecological fallacy, which occurs when people mistakenly assume that broad group averages apply to specific individuals within that group. “Relationships observed among countries cannot be assumed to apply to individuals,” Erşahin said. “A person living on relatively infertile soils is not inherently less intelligent than a person living on fertile soils, nor do the results imply that individual intelligence can be predicted from local soil conditions.”

“Readers should avoid interpreting the results in deterministic or normative terms,” he added. “The findings do not suggest that populations are constrained by their soils, nor that improving soil fertility alone would necessarily increase intelligence.” Erşahin reiterated that about 66 percent of the variation remained unexplained by their soil index.

“Factors such as education, healthcare, income, social conditions, culture, genetics, political stability, food imports, and agricultural practices are also important,” he explained. “For example, countries such as Singapore were identified as exceptions because they import much of their food and therefore are not strongly dependent on local soils.”

“Nevertheless, two countries with similar soil fertility can still differ substantially in IQ because of differences in education, public health, economic development, food imports, cultural factors, and other influences,” Erşahin noted. “Therefore, the results should be interpreted as evidence that soil fertility is one potentially important environmental correlate of human intelligence rather than a dominant or exclusive determinant.”

The soil index used in the analysis was also a simplified model. “The Soil Fertility Index used in this study is a simplified indicator based primarily on soil type and pH,” Erşahin said. “Although useful for global-scale comparisons, it does not capture many important dimensions of soil fertility and soil health, including organic matter content, nutrient concentrations, biological activity, land management practices, and food-system characteristics.”

“This study should be viewed as a starting point rather than a final conclusion,” Erşahin stated. “My long-term goal is to contribute to a more comprehensive understanding of how environmental conditions, particularly soil-related factors, may influence human development through food systems and nutrition.” The authors plan to refine their methods in future investigations.

“In future research, I hope to move beyond broad indicators such as soil type and pH and incorporate more direct measures of soil health and nutrient availability, including micronutrient status, soil organic matter, biological activity, and land management practices,” he added. “I am also interested in examining the pathways that may connect soils, crop nutritional quality, human nutrition, and cognitive development.”

“Another important objective is to collaborate with researchers from multiple disciplines, including soil science, nutrition, public health, geography, economics, psychology, and genetics, to develop more robust models capable of accounting for the many interacting factors that influence cognitive outcomes,” Erşahin explained.

“Ultimately, I envision this work contributing to a broader research framework that explores how environmental quality, food security, and human well-being are interconnected at regional and global scales,” he said. “Rather than asking whether soil fertility determines intelligence, a more productive long-term question is whether the quality of the environments that sustain food production can influence human development over generations, and if so, through what mechanisms.”

“Answering that question will require large-scale interdisciplinary collaboration and much more detailed data than are currently available,” Erşahin noted. “I hope readers understand that the purpose of this research was not to provide a definitive explanation for differences in intelligence among nations. Rather, it was to explore whether an overlooked environmental factor, soil fertility, might be part of a much larger system influencing human development.”

He views the work as an intriguing stepping stone for future research. “I believe the most exciting outcome of this research is not the correlation itself, but the opportunity to stimulate new interdisciplinary investigations into the relationships between environmental quality and human well-being,” Erşahin concluded.

The study, “Exploring geospatial link between soils and national intelligence quotient,” was authored by Sabit Erşahin, Faruk Tohumcu, and Rüştü Çallı. Erşahin is a professor of soil science at the Department of Soil Science and Plant Nutrition, Faculty of Agriculture, at Iğdır University.