“Intelligence is a polygenic feature in which several genes have genetic influence.”
We are the result of the constant influence of the environment on a genetic substrate, a process of constant change that lasts from conception to death. To understand how both forces determine a person’s intelligence, some researchers compare twins, adopted siblings and other family members. The idea is to go from those who have a common genetics but different environments (identical twins that were given up for adoption to two different families) to which the genetics are different but share an environment (two brothers, one of whom is at least adopted and who live in the same home, leading very similar lives). These studies show that the greatest similarity of intelligence quotient occurs between siblings that have the highest genetic agreement and, even more, if they share the same environment.
A surprising result is that the heritability of intelligence -the percentage of its variability in a given population that can be attributed to variation in the genes- increases steadily with age.
Heritability is less than 30% in pre-school children and reaches 80% at the end of adolescence, around 18-20 years, and is already maintained or even increases somewhat in adults of Western countries. In fact in adolescence, identical twins who have been raised apart answer the intelligence tests as if they were the same person while those adopted in the same home do it as if they were two strangers, without any connection between them. The conclusion is quite contrary to what instinct dictates: the vast majority of family environments are equally effective in developing intelligence, the adult’s intelligence quotient will be the same regardless of where or how he lives, how he was cared for or they educated, unless that environment was particularly sordid and inhuman; that is, with problems such as malnutrition, illness or abuse.
And why is this like this? Why does the power of the environment to model the intelligence quotient vanish and the genetic influences increase as the child becomes more independent? Studies on genetics and the environment offer some answers. One aspect to keep in mind is that heritability measures the proportion in the variation of a trait that can be attributed to the genes and not the proportion of the trait due to the genes. Therefore, if the environment changes and affects all members of a population equally, the average value of that trait, of intelligence for example, may rise, but the heritability remains the same because the variability has not increased. An example is height, the heritability of height is high but the average height does not stop increasing due to factors such as feeding and physical exercise, therefore it is a highly heritable trait but it can be strongly influenced by the environment. Second, a common mistake is to think that heritability does not change if the genes do not change. Since it is a ratio between genetic and environmental factors, if variation due to environmental causes increases, heritability decreases. The population of developing countries usually has more heterogeneous environments, often more unequal than in developed countries. That makes the heritability of many factors, also of intelligence, lower in these poor countries. Another example is the phenylketonuria that caused intellectual disability in everyone who had this genetic problem and, therefore, the heritability was 100%. At present, it can be prevented with a modified diet, so that its heritability has decreased. Finally, the fact that a trait is highly heritable does not imply that environmental factors such as teaching do not influence. The richness of the vocabulary, for example, is very inheritable and correlates to a large extent with general intelligence, although each word incorporated into the vocabulary is a learned experience. In a society with many available words in the environment, especially for individuals interested in incorporating them into their vocabulary, the number of words an individual learns depends to a large extent on their genetic predisposition and, therefore, heritability is high.
All children become active transformers of their own environment. All parents and educators have experienced some frustration when trying to take the child in a certain direction and simply see that it is not left. As he grows older, growing independence allows him to make decisions, allows him to choose the cognitive complexity of the environment he has around him and of which he is an active element. The brighter an individual thanks to his genetic endowment will choose cognitive situations and more demanding brain tasks, and will have more opportunities to reinforce his mental processes. Since a person’s ability to squeeze the environment in which he has lived is determined by the genes he treasures and since a better family atmosphere does not produce a general increase in intelligence quotient, it is not surprising that well-intentioned attempts to raise the intelligence quotients of a part of the disadvantaged population by improving the level of homes or the schools of that population have ended in a certain discouragement.
Apparently we can train any specific activity but not get a generalized change, a large increase in intelligence quotients. This does not imply that a good education, with resources and dedication, does not get anything. Early interventions achieve positive effects by reducing among other things the percentages of teenage pregnancies, delinquency and school dropout. It seems that we are not able to raise the intelligence quotient until it reaches similar levels to the average but we achieve something that is also important: to help these children learn more than they would if we did not support them and achieve more goals with the intelligence that they have of what they would achieve with that level and without that specific support.