A major new study reveals that different types of intelligence share distinct genetic links with a variety of mental illnesses. The study, published in Nature Communications, shows that genetic risk for diseases such as schizophrenia and bipolar disorder corresponds to lower problem-solving abilities and higher knowledge reserves. These findings indicate that the genetic relationship between mental health and cognition is highly specific to the particular mental skill in question.
Researchers have documented that many mental illnesses involve differences in cognitive abilities. People diagnosed with a particular disorder often score differently on standardized tests compared to unaffected people. Genetic data also points to underlying DNA overlap that influences both overall intelligence and mental health.
Until now, genetic research has typically treated cognitive function as a single, combined trait. This combined approach obscured the reality that human cognition is divided into entirely separate domains. Psychological research commonly divides intelligence into different categories with different developmental trajectories.
One category is reaction time. It measures basic processing speed and how quickly a person can respond to simple physical prompts. Fluid reasoning refers to the ability to solve new problems, identify patterns, and process complex information on the fly. Crystallized knowledge refers to the information, vocabulary, and facts that a person accumulates over a lifetime through education and cultural experiences.
Clinical observational studies show that mental illnesses typically affect these mental domains differently in daily life. People diagnosed with schizophrenia may experience difficulty with abstract, fluid reasoning tasks, but their crystallized knowledge of words and historical concepts is fully preserved. University of Texas at Austin researcher Diego Londono-Correa and his colleagues wanted to see if this pattern existed at the basic level of human DNA.
To investigate this, the research team analyzed data from a large genetic database. They looked at the genomes of hundreds of thousands of individuals. Through genome-wide association studies, they looked for small variations in human DNA that matched performance on specific cognitive tests.
Isolating the genetics of each specific mental skill required sophisticated statistical methods. The researchers created a mathematical model that categorizes genetic associations into different categories: reaction time, fluid reasoning, and crystallized knowledge. They logically ordered these traits from the most basic biological functions to the most socially dependent skills.
The researchers also isolated genetic factors associated with educational attainment that are not directly linked to traditional intelligence. They did this by mathematically subtracting genetic markers of pure cognitive ability from overall genetic markers of higher education. This left a genetic residue representing non-cognitive skills, including traits such as intellectual curiosity, persistence, and basic motivation to learn.
The researchers then compared these refined genetic profiles to known genetic risk factors for five major mental illnesses. These conditions include schizophrenia, bipolar disorder, autism spectrum disorder, attention deficit hyperactivity disorder, and Alzheimer’s disease. Genetic associations vary widely depending on the specific condition and precise cognitive skills.
Schizophrenia and bipolar disorder showed very similar genetic patterns to each other. Genetic risk for both conditions was associated with reduced basic processing speed and reduced fluid reasoning abilities. At exactly the same time, genetic risk for these two conditions correlated with more crystallized knowledge and better non-cognitive teaching skills.
Genetic risk for attention-deficit hyperactivity disorder showed a very different profile. This overlaps with slightly faster reaction times, which may reflect a genetic predisposition to rapid physical reactions that are less likely to be suppressed. This same genetic risk leads to lower fluid reasoning, lower crystallized knowledge, and lower overall non-cognitive educational skills.
Autism spectrum disorders showed genetic links with more highly crystallized knowledge. Genetic risk for Alzheimer’s disease, on the other hand, was completely separated by reduced fluid reasoning ability. Genetic risk for Alzheimer’s disease showed no statistical relationship with an individual’s crystallized knowledge.
During the expanded analysis, the researchers pinpointed 78 independent gene locations within the human genome that were associated with crystallized knowledge. Five of these gene locations had never been associated with cognitive traits in previous baseline studies. One of the newly identified gene locations is also known to affect bone density, highlighting how mutations in a single gene can have vastly different functions throughout the body.
Researchers mapped brain development across different life stages by looking at exactly where in the body these specific genes were active. They found that genes associated with fluid reasoning were most active in the brain during infancy and infant development. Genes tied to crystallized knowledge become more active during adolescence and early adulthood, reflecting how people acquire vocabulary and facts cumulatively over decades.
The research team also looked at local regions of the brain. Both types of intelligence showed intense genetic activity in brain tissues such as the frontal cortex. Fluid reasoning showed higher gene enrichment in the hippocampus compared to crystallized knowledge. The hippocampus is a physical brain structure deeply involved in memory formation and abstract problem solving.
The study also investigated how certain forms of intelligence overlap genetically with standard personality traits. Researchers found that openness to experience, a trait defined by general intellectual curiosity, is strongly genetically correlated with crystallized knowledge. Conscientiousness, which involves physical organization and daily discipline, was genetically associated with non-cognitive skills related to academic performance.
These overlapping genetic connections may help unravel long-standing evolutionary biology mysteries. Genetic mutations that increase the likelihood of experiencing severe mental illness have persisted in humans for thousands of generations. Evolutionary biologists refer to situations in which a single genetic variation causes both positive and negative biological effects as antagonistic pleiotropy.
Researchers suggest that some genetic variants that increase the risk of diseases such as bipolar disorder may also provide direct cognitive benefits. Genetic mutations that increase a person’s desire to learn, accumulate structural facts, and succeed in an educational environment may have tangible reproductive and social benefits. This evolutionary advantage may help explain why these genetic variations remain relatively common in human DNA today.
This study is characterized by limitations that affect how the results should be read and interpreted. The genetic data used in the analysis came almost entirely from individuals of European descent. This demographic limitation severely limits the extent to which specific findings apply to more globally diverse populations.
The researchers also noted that physical proximity between genetic variants on a single chromosome does not guarantee that they behave identically. The statistical methods implemented may combine nearby genetic variations into a single, cohesive group. This functional limitation may allow genetic signals that actually drive completely different biological mechanisms in the brain to combine by chance.
Future studies may extend this initial work by including highly diverse genetic databases representing different ancestries around the world. Scientists can also use animal models to advance laboratory experiments to understand exactly how these newly identified genetic locations change from birth to physical brain development. The authors emphasize that to fully understand the functioning of the human brain, it is absolutely necessary to view mental abilities as separate, discrete traits, rather than as a single, uniform score.
The study, “Crystallized and fluid cognitive abilities have different genetic associations with neuropsychiatric disorders,” was authored by Diego Londono-Correa, Javier de la Fuente, Gail Davis, Simon R. Cox, Ian J. Deary, K. Page Harden, and Elliott M. Tucker Drob.
