February 11, 2025, 3:52 pm | Read time: 4 minutes
Previous studies have shown that a lower intelligence test score (ITS) tends to be associated with a higher body mass index (BMI) and an increased risk of obesity. However, with obesity on the rise worldwide, the question arises: Has this correlation changed over the decades? A new Danish study investigated this.
Obesity rates have risen worldwide in recent years.1 At the same time, there has long been evidence that people with lower intelligence test scores are more likely to be obese.2 Earlier research from Denmark suggested that the link between obesity and intelligence may have been stronger in the past than it is today and has weakened with rising obesity rates.
Overview
Previous Findings on Obesity and Intelligence
Previous research with Danish conscripted men showed an inverse relationship between intelligence and BMI. Intelligence scores peaked at a BMI of around 20 and decreased with higher BMI values.3 However, researchers now wondered whether this pattern had changed in later cohorts – particularly in the context of rising obesity rates. It was thought that the link between low intelligence and higher BMI may have weakened, as obesity is now more common than it used to be.
Tests with Conscripted Men
The study investigated whether the relationship between intelligence and obesity has changed over the decades. Data from a total of 419,319 young men born in Denmark between 1939 and 1959 and between 1983 and 2001 were analyzed – an earlier cohort from the Danish conscription database and a later cohort from the Danish conscription register.4
All participants underwent a standardized examination as part of the draft for military service at the age of around 18 to 27. In this, their height, weight, and intelligence were tested, among other things. The intelligence test called “Børge Priens Prøve” was the same in both cohorts and comprised four subtests:
- Letter matrices
- Verbal Analogies
- Number Sequences
- Geometric Figures
Higher BMI and Lower Intelligence Scores Are Linked
The researchers observed an increase in BMI between the earlier and later cohorts: in the first group, the average value was 21.7, while in the second, it was 23.5. The obesity rate also increased dramatically: in the early cohort, 0.8 percent were obese, while in the later cohort, the rate was 6.7 percent. The prevalence of overweight (including obesity) increased from 7.9 percent to 21.3 percent.
Despite these changes, the relationship between intelligence and BMI remained stable in both cohorts. People with lower intelligence scores had a higher risk of obesity and being overweight in both groups – regardless of when they were born. The intelligence test score continued to show an inverse relationship with BMI: it reached its maximum at a BMI of around 20 and decreased with increasing BMI. In the late cohort, however, this decline was somewhat less drastic than in the early cohort.
Interpreting the Study’s Findings
The results of the study show that the relationship between intelligence and BMI has remained constant over the decades despite the dramatic increase in obesity rates. This means that the increase in overweight and obesity has not led to people with higher intelligence being more affected by obesity.
Also, the fact that this relationship occurs in both cohorts speaks to underlying biological or behavioral mechanisms that have kept this relationship stable over the decades. The results suggest that there are factors that influence both intelligence and BMI, possibly via genetic, social, or educational mechanisms.
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Classification of the Study
The study is characterized by its large sample size. By using the same test procedures and standardized BMI measurements in both cohorts, the results can be considered largely reliable.
Nevertheless, there are some limitations. For example, the study only includes men, so the results are not transferable to women. The fact that the test subjects only came from Denmark does not ensure that the results would also apply to other population groups. In addition, people with certain illnesses who were excluded from military service were not taken into account. This could have led to distortions, particularly in the late cohort. Furthermore, this study provides no evidence of a causal relationship between BMI and intelligence.