It's not easy to understand where intelligence comes from. Genetics and environment seem closely linked.
Thus, a team of researchers from the Beijing Genomics Institute (BGI), the largest genetic research center in the world based in Beijing, is trying to identify the genes of intelligence. Two thousand two hundred genius, holders of an intelligence quotient (IQ) out of the ordinary (145 or more), were recruited all over the world and their DNA was taken. By studying their genome, researchers hope to identify genetic variants associated with above-average intellectual abilities. A titanic task, when we know that the human genome is made up of around 25,000 genes, each made up of a few tens of thousands of pairs of nucleotides…
Despite its imperfections, the IQ is the most powerful indicator
This spectacular initiative is only the latest episode in a quest that has driven researchers for more than a century: to understand how the brain produces intelligence. Is it first of all a story of genes, or does the environment — that is to say both the social level of the family, education… — play a decisive role ?
To answer these questions, we must first know what we are measuring. Nowadays, intelligence is generally defined as the ability to acquire, use, and relate knowledge in tasks involving problem solving or adapting to new situations. But the majority of researchers agree that we still do not know what exactly this term covers. Hence the difficulty in developing relevant tools to measure and quantify intelligence. Despite its imperfections, and the criticisms of some who denounce its reductive vision, IQ remains the most widely used performance indicator. An average IQ is 100. And 95% of any given population has an IQ between 70 and 130.
Looking at their results, there is little doubt about the genetic origin of part of intelligence. A large majority of twin studies show that monozygotes – “identical” twins, who have the same genome – have closer IQs than “fraternal” twins or children from the same family.
On the other hand, researchers believe that there is no single gene for intelligence. Rather multiple nucleotide mutations. At the forefront of this research, the behavioral geneticist of the Institute of Psychiatry in London, Robert Plomin, estimates that the genes involved in intelligence are very numerous (hundreds, perhaps thousands), each with a weak effect . According to this researcher, there are probably no specific genes for verbal skills while others are involved in spatial skills. Genes linked to intelligence would be expressed in a vast cerebral network, and would produce different effects depending on the region where they are expressed.
A disadvantaged social environment would result in lower results at school
It remains to list them. However, while research on intellectual disability (IQ less than 70) has identified more than 300 genes whose mutations are associated with different forms of mental retardation, the search for genetic modifications explaining "normal" variations in IQ s turned out to be disappointing. A few genes were indeed associated with IQ, but the population samples were too small, and the studies were not sufficiently cross-validated.
The difficulty in going further also comes from the complexity of the links between environment and genetics. Because, today, the idea that the cognitive performance of an individual depends more or less equally on his genetic heritage and his environment is a consensus. Numerous studies have thus established a link between the socio-economic level of parents and the cognitive performance of children. A disadvantaged social environment would result in particular in lower results at school.
In question : the level of education of course, but also the stress, the nutrition or the availability of the parents. To establish the effects of the environment on intelligence, a team from the University of Southern California followed a cohort of 1,099 individuals between the ages of 3 and 20. She thus highlighted a link between the income of the parents, the morphology of the brain of the children, and their results on IQ tests. The brain surface – the convolutions of the outer layer of the cortex – of children living in families with the lowest incomes was up to 6% smaller than that of children from advantaged families. And these variations were particularly notable in the brain regions in charge of language, reading, decision-making functions and spatial orientation. Numerous studies on rodents have also shown that stimulating environments with motor activities, multiple social interactions, treasure hunts to find food and the regular introduction of new objects boost their intelligence.
But where things get complicated is that the work of psychologist Eric Turkheimer, of the University of Virginia, carried out on hundreds of homozygous twins demonstrates that the effect of genes is itself dependent on socio-economic status. of the family. Thus, in children from wealthy families, about 60% of differences in IQ would be linked to genes. Conversely, for children from underprivileged backgrounds, the genetic influence seems to weigh less : the environmental constraints are so strong that they could influence the development of intelligence and the expression of genes from intrauterine life.
And environmental factors are not just limited to socio-cultural elements. Each of us can experience that we perform better when we are in a good mood and not stressed. As soon as adults are warm and benevolent, the child secretes brain molecules (oxytocin, dopamine, endorphins, serotonin) that make him happy, sociable, peaceful, motivated and creative. His brain is developing favorably. He memorizes and learns better. In short, he is smarter.
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