A study of Dutch twins has uncovered a slight association between higher intelligence and a reduced risk of psychopathology, primarily driven by common genetic factors. This means that the same genetic influences that contribute to higher intelligence also appear to protect against the development of certain mental health issues. Notably, the heritability of anxiety and negative affect—traits associated with mood disorders like depression—was found to be greatest in individuals with below-average intelligence. This study was published in the journal Behavior Genetics.
Intelligence, the ability to learn, understand, and apply knowledge to solve problems, encompasses various cognitive functions such as reasoning, memory, and decision-making. It manifests in many different areas of life and in many different forms. Intelligence is not limited to academic knowledge but also includes the capacity to adapt to new situations and environments across all domains of life.
Previous studies have consistently shown a negative association between intelligence and psychopathology. This means that individuals with lower intelligence scores are generally at a higher risk of developing various mental health issues, including anxiety, depression, and behavioral disorders. Despite this well-documented correlation, the reasons behind it remain a subject of ongoing research. Scientists have been particularly interested in understanding whether this relationship is driven by genetic factors, environmental influences, or a combination of both.
To explore this relationship further, study author Susanne Bruins and her colleagues examined the link between intelligence, as assessed by psychological tests, and five aspects of psychopathology in 7-year-old twins. The five aspects of psychopathology they focused on were: negative affect (including depressive symptoms and withdrawn behavior), anxiety (encompassing anxiety- and phobia-related symptoms), oppositional defiant disorder (which involves disobedient and defiant behavior), autism (referring to problems with communication, affect, and flexibility), and attention-deficit hyperactivity disorder (ADHD), which includes attention problems, hyperactivity, and impulsive behavior.
The researchers utilized data from the Young Netherlands Twin Register, a long-term study that recruits twins at birth and follows them throughout their lives. Parents of the twins regularly complete surveys about their children’s development and behavior. This register, which was initiated in 1986, provided the researchers with a valuable source of longitudinal data.
For this study, the researchers focused on a subgroup of 1,089 twins from the register, specifically those for whom both intelligence test scores and detailed psychopathology data were available. This subgroup included 543 complete twin pairs, with 262 pairs of monozygotic (identical) twins and 281 pairs of dizygotic (fraternal) twins. Monozygotic twins share 100% of their genetic material, while dizygotic twins share about 50%, similar to non-twin siblings. This difference in genetic similarity allowed the researchers to make inferences about the relative contributions of genetic and environmental factors to both intelligence and psychopathology.
The intelligence of the twins was measured using a range of age-appropriate IQ tests, including the Revised Amsterdam Child Intelligence Test, the Wechsler Adult Intelligence Scale (WAIS), and the Wechsler Intelligence Scale for Children (WISC), depending on the age of the child at the time of testing. Psychopathology was assessed using the Child Behavior Checklist (CBCL), a widely used tool that identifies symptoms of various mental health conditions in children. The CBCL is designed to be sensitive across a wide range of intellectual abilities, making it a suitable instrument for this study.
The study revealed that all five groups of psychopathology symptoms analyzed—negative affect, anxiety, oppositional defiant disorder, autism, and ADHD—were slightly less common in participants with higher intelligence. Although the association was statistically significant, it was very slight, indicating that intelligence alone is not a strong predictor of mental health outcomes.
One of the most interesting findings was that the association between intelligence and symptoms of anxiety, ADHD, and autism was primarily driven by common genetic factors. This suggests that the same genetic influences that contribute to higher intelligence also reduce the risk of developing these mental health issues. However, the relationship between intelligence and anxiety or oppositional defiant disorder did not appear to be driven solely by genetic factors, indicating that other environmental or developmental influences might be at play.
The study also found that the heritability of anxiety and negative affect varied depending on the level of intelligence. Specifically, the heritability of both anxiety and negative affect was highest in participants with below-average intelligence. This means that genetic factors contributing to these conditions are more pronounced in children with lower intelligence scores. In contrast, environmental factors appeared to play a more significant role in children with higher intelligence, particularly in the development of anxiety.
“We found that intelligence correlated negatively with negative affect, anxiety, ODD, ADHD, and autism. These correlations in part reflected common genetic effects, with genetic factors that increase intelligence decrease psychopathology. Genetic and environmental effects on negative affect and anxiety (respectively) were moderated by intelligence, such that the heritability of both anxiety and negative affect was greatest in children with lower IQ [intelligence quotient],” the study authors concluded.
The study sheds light on the factors behind the link between psychopathology and intelligence. However, it should be noted that the association between intelligence and psychopathology was very low, practically negligible, and that the study was conducted on a selected group of twins—those who had all the necessary data in the register. Given these very weak associations, it is possible that the findings might not be present if the study were conducted on a less selected sample of twins.
The paper, “Are Genetic and Environmental Risk Factors for Psychopathology Amplified in Children with Below‑Average Intelligence? A Population‑Based Twin Study,” was authored by Susanne Bruins, Elsje van Bergen, Maurits W. Masselink, Stefania A. Barzeva, Catharina A. Hartman, Roy Otten, Nanda N. J. Rommelse, Conor V. Dolan, and Dorret I. Boomsma.
