According to many recent studies, genetics plays a bigger role in making someone smart or not than what was initially expected or hoped for.
Several studies with twins2 and studies with fraternal siblings have identified intelligence as one of the highest heritable traits.3
In other words, just as someone with tall parents is likely to be tall as well, someone with smart parents is also likely to be more intelligent.
This relationship between heredity and intelligence has been a somewhat controversial topic due to two main reasons: (1) it could lead to discrimination, particularly if it is used to segregate races and genders, and (2) it hinders the notion of freedom and the “self-made man”.
After all, if everything is already decided from birth, why bother? If I am born more intelligent than the others there is no point in making an effort, and if I am not the sharpest tool in the shed, then I would be spending my life trying to catch up to others, without any hopes of getting to the top.
Although genetics seem to play a big role in intelligence, it is not a sufficient explanation for why some people are smarter than others. A person may be born with the predisposition to be intelligent, but that does not mean that this predisposition will be transposed to their daily reality. Several factors can contribute to or negatively affect the development of intelligence.
The example of height is pertinent in this case, as this is also a highly heritable trait. Tall parents have a higher probability of having tall offspring but the real height of the kids will depend on other factors such as diseases and nutrition. A malnourished child is less likely to grow tall, regardless of their genetic predisposition.
The truth behind why some people are smarter than others seems to lie somewhere in the relationship between genetics and environmental factors.
For example, although studies with twins have been used to prove the importance of genetics in the development of intelligence, they also revealed that the IQs of the siblings are less similar when they are raised separately and in different environments.4
Likewise, some studies have shown that firstborns tend to have a higher IQ than their siblings, most likely because the parents dote them with more attention and stimulation.5
Education is also consistently linked to IQ increases.6
The Flynn effect should also be taken into account. Even if it has come under scrutiny and criticism in recent years, many researchers still find its base arguments pertinent and provable.7
According to the Flynn effect, the consistent increase in IQ scores for the general population after 1900 can be linked to better healthcare, nutrition, and access to education, and not necessarily to genetics.
Regardless, it is very hard to pinpoint exactly what makes someone smarter than others or at least the reason for that difference.
For example, let us say that A is genetically predisposed to be more intelligent than B, but B is keen on improving his IQ and pursues higher education, has a healthy lifestyle, and enjoys stimulating his brain. The result could be that even if A was initially smarter than B, B has overtaken A in the course of his life.
It seems clear the answer to why some people are smarter than others is a bit more complex than one might think at first. Genetics do play a big role in making someone more predisposed or not to be intelligent, but the environmental factors cannot be dismissed either.
Even if someone is born with a higher chance of being smart, their education, health, and nutrition will affect the overall cognitive development of the individual.
1 Carson, J. (2015). Intelligence: History of the Concept. In Wright, James D. (Editor), International Encyclopedia of the Social & Behavioral Sciences (Second Edition), Elsevier, Pages 309-312. https://doi.org/10.1016/B978-0-08-097086-8.03094-4.
2 Willoughby, Emily A. et al. (2021). Genetic and environmental contributions to IQ in adoptive and biological families with 30-year-old offspring. Intelligence, Volume 88, https://doi.org/10.1016/j.intell.2021.101579; Briley, D. A., & Tucker-Drob, E. M. (2013). Explaining the increasing heritability of cognitive ability across development: a meta-analysis of longitudinal twin and adoption studies. Psychological science, 24(9), Pages 1704–1713. https://doi.org/10.1177/0956797613478618
3 Plomin, R., & Deary, I. J. (2015). Genetics and intelligence differences: five special findings. Molecular psychiatry, 20(1), Pages 98–108. https://doi.org/10.1038/mp.2014.105
4 Oommen, A. (2014). Factors Influencing Intelligence Quotient. J Neurol Stroke 1(4): 00023. DOI: 10.15406/jnsk.2014.01.00023
5 Lehmann, J-YK, Nuevo-Chiquero, A., Vidal-Fernandez, M. (2018). The early origins of birth order differences in children’s outcomes and parental behavior. J. Human Resources, Winter 2018, vol. 53 no. 1, Pages 123-156. DOI:10.3368/jhr.53.1.0816-8177
6 Ritchie, S. J., & Tucker-Drob, E. M. (2018). How Much Does Education Improve Intelligence? A Meta-Analysis. Psychological Science, 29(8), 1358–1369. https://doi.org/10.1177/0956797618774253
7 Trahan, L. H., Stuebing, K. K., Fletcher, J. M., & Hiscock, M. (2014). The Flynn effect: a meta-analysis. Psychological bulletin, 140(5), 1332–1360. https://doi.org/10.1037/a0037173