A massive new study reveals that different types of intelligence share distinct genetic links with various psychiatric conditions. The research, published in Nature Communications, shows that genetic risks for conditions like schizophrenia and bipolar disorder correspond with lower problem-solving abilities but higher accumulated knowledge. These findings demonstrate that the genetic relationship between mental health and cognition is highly specific to the particular mental skill in question.
Researchers have documented that many psychiatric conditions involve differences in cognitive performance. People diagnosed with certain disorders often score differently on standardized tests compared to unaffected individuals. Genetic data has also pointed to an overlap in the underlying DNA that influences both overall intelligence and mental health.
Previous genetic research typically treated cognitive function as a single, combined trait. This combined approach masked the reality that human cognition is divided into totally separate domains. Psychological research commonly splits intelligence into distinct categories with disparate developmental trajectories.
One category is reaction time, which measures basic processing speed and how quickly a person can respond to simple physical prompts. Fluid reasoning represents the ability to solve novel problems, identify patterns, and process complex information on the spot. Crystallized knowledge refers to the information, vocabulary, and facts a person accumulates over a lifetime through education and cultural experience.
Clinical observational studies show that psychiatric conditions usually affect these mental domains differently in daily life. Individuals diagnosed with schizophrenia might experience challenges with abstract fluid reasoning tasks but fully retain their crystallized knowledge of words and historical concepts. Diego Londono-Correa, a researcher at the University of Texas at Austin, and his colleagues wanted to see if this pattern existed at the foundational level of human DNA.
To investigate this, the research team analyzed data from massive genetic databases. They examined the genomes of hundreds of thousands of individuals. Through a genome-wide association study, they looked for tiny variations in human DNA that corresponded with performance on specific cognitive tests.
Isolating the genetics of each specific mental skill required an advanced statistical technique. The researchers created a mathematical model that separated the genetic associations into distinct categories for reaction time, fluid reasoning, and crystallized knowledge. They sequenced these traits logically from the most basic biological functions to the most socially dependent skills.
The team also isolated genetic factors related to educational attainment that are not tied directly to traditional intelligence. They did this by mathematically subtracting the genetic markers of pure cognitive abilities from the overall genetic markers of high educational achievement. This left a genetic remainder that represents noncognitive skills, which encompass traits like intellectual curiosity, persistence, and the baseline motivation to learn.
The researchers then compared these refined genetic profiles against the known genetic risk factors for five major psychiatric conditions. These conditions included schizophrenia, bipolar disorder, autism spectrum disorder, attention deficit hyperactivity disorder, and Alzheimer’s disease. The genetic associations varied heavily depending on the specific condition and the exact cognitive skill.
Schizophrenia and bipolar disorder showed very similar genetic patterns to one another. The genetic risk for both conditions was linked to lower basic processing speed and lower fluid reasoning capacity. At the exact same time, the genetic risk for these two conditions correlated with higher crystallized knowledge and greater noncognitive educational skills.
The genetic risk for attention deficit hyperactivity disorder displayed a completely different profile. It overlapped with a slightly faster reaction time, which might reflect a genetic tendency toward less inhibited, rapid physical responses. This same genetic risk linked to lower fluid reasoning, lower crystallized knowledge, and lower overall noncognitive educational skills.
Autism spectrum disorder showed a genetic association with higher crystallized knowledge. Meanwhile, the genetic risk for Alzheimer’s disease was entirely isolated to lower fluid reasoning capacity. The Alzheimer’s genetic risk showed no statistical relationship with a person’s crystallized knowledge.
During their extended analysis, the researchers pinpointed 78 independent genetic locations in the human genome associated with crystallized knowledge. Five of these genetic locations had never been linked to any cognitive trait in previous baseline research. One of the newly identified genetic locations is also known to influence bone mineral density, highlighting how single gene variations can perform vastly different functions across the body.
By looking at exactly where these specific genes are active in the physical body, the researchers mapped brain development across different life stages. They found that the genes associated with fluid reasoning are most active in the brain during early childhood and infant development. Genes tied to crystallized knowledge become much more active during adolescence and early adulthood, mirroring how people acquire vocabulary and facts cumulatively over decades.
The research team also looked at localized regions of the brain. Both types of intelligence showed heavy genetic activity in brain tissues like the frontal cortex. Fluid reasoning showed higher genetic enrichment in the hippocampus compared to crystallized knowledge. The hippocampus is a physical brain structure heavily involved in memory formation and abstract problem-solving.
The study also examined how specific forms of intelligence genetically overlap with standard personality traits. The researchers found that openness to experience, a trait defined by general intellectual curiosity, was strongly genetically correlated with crystallized knowledge. Conscientiousness, which involves physical organization and daily discipline, was genetically linked to the noncognitive skills associated with academic achievement.
These overlapping genetic links might help illuminate a longstanding evolutionary biology mystery. Genetic variants that increase the likelihood of experiencing severe psychiatric conditions have persisted in the human population for thousands of generations. Evolutionary biologists refer to situations where a single genetic variation causes both positive and negative biological effects as antagonistic pleiotropy.
The researchers suggest that some genetic variations that increase the risk for conditions like bipolar disorder might simultaneously offer direct cognitive advantages. If a genetic variant boosts a person’s drive to learn, accumulate structural facts, and succeed in educational environments, it could provide a tangible reproductive or social advantage. This evolutionary benefit might help explain why these genetic variations remain relatively common in human DNA today.
The study does feature 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 ancestry. This demographic restriction strictly limits how well the specific findings might apply to more globally diverse populations.
The researchers also noted that physical proximity between genetic variations on a single chromosome does not guarantee they behave identically. The statistical methods implemented sometimes merge nearby genetic variations into a single cohesive grouping. This functional limitation could accidentally combine genetic signals that actually drive entirely different biological mechanisms in the brain.
Future research could expand upon this initial work by including highly diverse genetic databases representing different global ancestries. Scientists could also pursue laboratory experiments, using animal models to understand exactly how these newly identified genetic locations alter physical brain development from birth. The authors emphasize the absolute necessity of looking at mental abilities as distinct, separate traits rather than one uniform score to fully capture human brain function.
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 Davies, Simon R. Cox, Ian J. Deary, K. Paige Harden, and Elliot M. Tucker-Drob.