The nature-versus-nurture debate doesn’t get much more febrile than in the domains of intelligence and mental illness.
When it comes to the smarts, education and healthy living rate high on the nurture side of the ledger, as does the “Flynn effect” – which suggests intelligence, like a muscle, can “bulk up” in the face of cognitive demands. In mental illnesses such as depression and anxiety, the triggering roles of early life abuse, dysfunctional family and substance misuse are well described.
But genetics plays its part, a role better understood now thanks to two large studies published in the journal Nature Genetics. They identify a host of genes linked to intelligence and psychological disorders, the brain changes that might cause them, and further relationships with illnesses including Alzheimer’s disease and attention deficit hyperactivity disorder (ADHD).
Both studies were led by Danielle Posthuma from the Centre for Neurogenomics and Cognitive Research at VU University Amsterdam, in the Netherlands.
In the first, the team analysed genome wide association studies involving nearly 270,000 people. These compared genetic changes called single nucleotide polymorphisms (SNPs) with a range of indicators of intelligence taken from US college admission scores (known as SAT), IQ tests, and other measures of logic, verbal and mathematical reasoning, and memory.
The researchers identified no fewer than 939 new genes associated with intelligence. In a separate analysis, they linked intelligence to genes responsible for growth of brain cells as well as regulation of the structure and activity of synapses, the junctions between brain cells through which they “talk” to each other.
The findings also add to knowledge of the relationship between intelligence and a range of illnesses.
The data showed that as intelligence goes up, the incidence of ADHD, depression, Alzheimer’s disease and schizophrenia went down, suggesting a protective effect. Higher intelligence was linked to longevity but also, on the downside, to autism. Critically, the link between genes for intelligence and those regulating brain structure could explain its relationship to the various disorders.
“Our results indicate overlap in the genetic processes involved in both cognitive functioning and neurological and psychiatric traits and provide suggestive evidence of causal associations that may drive these correlations,” the authors conclude.
The second study focused on the genetic basis of neuroticism, one of the so-called “big five” personality traits, marked by moodiness, self-consciousness and a propensity to anxiety and depression – Freud’s classic “neuroses”.
Neuroticism is also a risk factor for schizophrenia. The researchers crunched data on about 450,000 people, retrieved from sources including the UK Biobank study and the home genetic testing company 23andMe.
They found nearly 600 genes associated with neuroticism, specifically depression, lowered mood and worry. They showed those genes are linked to others that influence brain cell growth and the production of dopamine and serotonin, chemical messengers essential for normal function but also implicated in mental illnesses such as depression and schizophrenia.
The authors stress that “the genetic architecture of neuroticism is extremely polygenic,” but also say that, in relation to depressed mood and worry, genes present “distinct causal mechanisms”.
In that perennial debate, then, the nature side of the ledger would seem to have just got heavier.