The greatest minds have yet to resolve fully one of the most intriguing questions about human development: to what extent do genes or environmental factors determine our intelligence?
The debate has courted controversy since the late 19th century when psychologists first began to study the subject. More recently, some scientists have even suggested a third factor may be in play: luck.
In September, the New Scientist reported on research into an influence termed ‘randomness’. This suggests that, while genes and environment have a role in forming us, there are chance events, which happen in the womb that shape our brains. As the writer of the piece, Clare Wilson, put it: “We should see ourselves as the product of nature, nurture and ‘noise’.”
In the case of genetics, how do scientists go about measuring its role in intelligence? When assessing a given trait, some researchers make estimates of ‘heritability’ and assign a value ranging from 0 to 1. A 2006 study, for example, found that adult height has a heritability estimated at 0.80 when looking at height variability within families where the environment is very similar – primarily genetic, in other words. As you might expect, intelligence is a more complex matter, with more than one gene coming into play; in fact, it requires at least 500 genes.
Incidentally, a rather surprising study which found a link between height and intelligence – with tall people tending to be more intelligent on average – is perhaps best set aside for discussion another day.
Other research relies on using intelligence quotient (IQ) as a measure, with elements of intelligence including the ability to solve problems, understand complex ideas and learn from experiences. For more than a century, twins have provided researchers with ideal case studies. Discredited research aside, numerous studies involving twins suggest that between 40% and 80% of the variance in IQ is linked to genetics, suggesting that genes may play a larger role than environmental factors in determining a person’s IQ.
Today, psychologists tend to recognise that both genetics and the environment are involved in determining intelligence, and their focus now is on exactly how much of an influence each factor has. For example, a person born with high intelligence may not score well in IQ tests for a variety of non-genetic reasons, such as growing up in a deprived environment or lacking access to mental stimulation as a child.
On the BBC’s Science Focus website, Dean Burnett raises another thorny issue – the definition of intelligence. He writes: “While the dictionary definitions are fairly straightforward, describing intelligence as, for example, ‘the ability to acquire and apply knowledge and skills’, the scientific consensus on what intelligence actually is, regarding how it should be defined and assessed, is still disputed.”
Burnett also poses the question of “how many” intelligences a human possesses – is it one general intelligence that covers all circumstances or several different types of intelligence for different tasks, such as musical, spatial, linguistic, and so on?
When reviewing articles on the subject of intelligence, prepare to encounter a raft of online debates sparked by numerous scientific studies. New research is also bringing other theories to the table, such as the study of randomness mentioned earlier, and the possibility that children inherit cleverness from their mothers. But as 2023 gets into its stride, with all the challenges it will bring, perhaps the important question is what we can do with the intelligence that nature, nurture – or chance – has given us.
References
Nature, nurture, luck: Why you are more than just genes and upbringing, New Scientist, 21 September 2022
Assumption-Free Estimation of Heritability from Genome-Wide Identity-by-Descent Sharing between Full Siblings, PLoS Genet. 2006 Mar; 2(3): e41.
Should I blame my parents for my IQ?, Dean Burnett, BBC Science Focus, 24 January 2022
