Intelligence predicts health and mortality – but why?
New research reveals some genes are implicated in both.
Key facts:
- Intelligence test scores predict mortality, illnesses, and health behaviors. Higher scores indicate lower risk.
- Both intelligence and health/disease outcomes are highly polygenic – many genetic variants of small effect contribute.
- Genetic correlations exist between intelligence and health outcomes like longevity, Alzheimer’s, cardiovascular disease, lung function, obesity, and more.
- Causal relationships and directions are still unclear. Intelligence may impact health via behaviors or social factors like education. Pleiotropic genes could independently affect intelligence and health.
- New methods like polygenic scoring and Mendelian randomization can further elucidate genetic overlaps and causal relationships.
Source: Current Opin Psychol 2019
Intelligence Linked to Health Outcomes
For decades, studies have shown that intelligence test scores are predictive of health outcomes later in life.
Scores from childhood strongly predict mortality from all causes, with higher intelligence linked to lower risk.
The associations remain robust even after adjusting for childhood socioeconomic status, and are only partly explained by adult education and socioeconomic status.
In addition to mortality, higher intelligence also predicts lower risk for illnesses like cardiovascular disease, hypertension, stroke, and lung cancer.
The reasons behind the intelligence-health link have been unclear.
Healthier behaviors, higher education levels, and increased affluence among more intelligent individuals may play a role.
But new findings in genomics suggest that part of the association has genetic underpinnings.
The Genetic Underpinnings of Intelligence and Health
Both intelligence and disease/health outcomes are highly polygenic traits, meaning they are influenced by many genetic variants of small effect spread across the genome.
Early candidate gene studies of intelligence yielded inconsistent results.
But genome-wide association studies (GWAS) with sample sizes in the hundreds of thousands have now identified over 150 genomic regions associated with intelligence test scores.
Many of these regions harbor genes previously linked to physical and mental illnesses.
Using genomic methods like GREML analysis in the GCTA framework, SNP heritability of intelligence is estimated around 25%, about half of the 50-70% estimated from twin studies.
This gap is likely due to causal variants not well tagged by common SNPs genotyped on GWAS arrays.
New statistical genetics methods are helping close this “missing heritability” gap.
Likewise, GWAS have uncovered polygenic architectures underlying diseases, biomarkers, and health behaviors.
Coronary artery disease, blood pressure, cholesterol levels, lung function, body mass index, smoking, Alzheimer’s, and more are influenced by hundreds of SNPs.
The additive effects of these variants contribute to heritable variation.
Genetic Links Between Intelligence and Health
With intelligence and health outcomes both showing polygenic genetic contributions, researchers have started examining whether there are shared genetic influences.
New methods like bivariate GREML, LD regression, and polygenic scoring have found genetic correlations between intelligence and:
- Longevity and parental age at death
- Coronary artery disease
- Blood pressure and cholesterol
- Alzheimer’s disease
- Intracranial volume
- Birth weight and height
- Lung function
- Body mass index and obesity
- Smoking behaviors
- Happiness and health satisfaction
- Nearsightedness and farsightedness
Though suggestive of shared genetic factors, these correlations alone don’t reveal causal relationships or directionality.
Studies finding associations between polygenic risk scores for health outcomes and intelligence provide further evidence of genetic overlap.
But how and why do these genetic links exist?
Possible Explanations for the Genetic Links
There are several possibilities that could explain the apparent genetic correlations between intelligence and health:
- Pleiotropic genes independently influence both intelligence and health traits. Biological pleiotropy describes when one gene affects multiple phenotypes. For example, a gene implicated in neural development may also impact cardiovascular system development. Pleiotropic genes affecting tissue integrity throughout the body could contribute to associations.
- Causal effects of intelligence on health due to behavioral and social mechanisms. If genes influencing intelligence confer benefits like healthier behaviors, more education, higher health literacy and socioeconomic status, this could lead to better health. Intelligence may be a protective factor against disease.
- Reverse causation: causal effects of health on intelligence. Early-life health conditions affected by genetics could shape neural development and intelligence. Chronic diseases that impact the brain may lower cognitive performance.
Studies employing methods like Mendelian randomization and bidirectional modeling can provide evidence for causality and directionality not available from genetic correlation data alone.
Early MR studies found no clear causal relationships between intelligence and BMI, blood pressure, coronary artery disease, or diabetes.
But larger analyses have provided some evidence for causal effects of intelligence on intracranial volume, Alzheimer’s disease, BMI, and smoking.
The relationships are complex and warrant further scrutiny.
Beyond intelligence, health outcomes also correlate genetically with related traits like education and socioeconomic status.
These variables link to both intelligence and health, and may represent mediating pathways.
Disentangling the contributing roles of genetics and environmental mechanisms remains an active area of research.
Further Research: Genetics, Health/Disease, Longevity
In summary, a web of genetic links exists between intelligence and health, likely reflecting both biological pleiotropy and causal pathways.
The connections represent an exciting frontier in understanding lifelong health and cognitive aging.
As genome-wide analyses continue to scale up, and methods for investigating causality improve, scientists are gaining clarity on the genetic interplay between mind and body.
However, many open questions remain regarding the mechanisms, directionality, confounders, and interplay between genetic and environmental factors.
Uncovering the role genetics plays in the intelligence-health relationship has profound implications for medicine, public health, education, and social policy aiming to improve human health and lifespan.
The intricate genetic links between intelligence and health illustrate the deeply interconnected nature of human biology.
References
- Study: What genome-wide association studies reveal about the association between intelligence and physical health, illness, and mortality
- Authors: Ian J Deary et al. (2019)
