Wait, what is genetics?

Imagine that you are walking down the street and you overhear a conversation: “Bobby has been extremely smart since he was a small boy. It is in his DNA”. This expression has become so common that we immediately understand that it refers to a key characteristic of Bobby. We also accept quite easily that our genes can influence our intelligence, but how? And to which extent? 

To address these questions we are going to take a brief stop and start with a more basic issue: what is DNA? Well, we can think about it as an instruction manual, one better than those of Ikea. This manual contains all the necessary information for an organism to develop, survive, and reproduce. Importantly, it carries the ability to copy and pass information to the following generations. Segments of this DNA, called genes, are the ones that specify eye color or blood type. 

Now, if DNA is a textbook, and genes are like the chapters in our novel, the letters that compose them would be nucleotides. There are 4 different types, 4 letters, that always go in pairs. Their combination constitutes the code of life. Each of our cells contains a full copy of our DNA and 99,9% of this genetic material is common in every person. It is what makes us humans. This means that only 0.1% of DNA is unique, accounting for our individual characteristics.

Genetics visualization
Looking into our DNA

Although it is clear that genetics has an important role in physical appearance or the heritability of certain disorders, a question that has been present for decades is whether our genetic code could affect more complex features such as intelligence.

Genetics and intelligence

Picture a group of people you consider very smart. It could be your mother, Stephen Hawking, or Da Vinci. If we differ in just 0.1% of our DNA, it should be easy to look at them and see if there is a magic combination to be a prodigy, right? Well, for starters this small percentage corresponds to nothing less than 3 million nucleotides, 3 million of our “letters” that have to be analyzed. Moreover, intelligence is challenging to study, partly because it can be described and measured in different ways. 

After researchers agreed to a general definition –the ability to learn from experiences and adapt to changing environments, evaluated across different tests– they found that there is no such thing as a “smart gene”. Although the genetic code has a measurable impact on our intelligence, the relationship is not as simple as inheriting a couple of DNA fragments and being a genius. Intelligence is determined by a very large number of interacting genes whose effects are small and cumulative. This means that although each of them, isolated, has little influence, when summed together their impact becomes significant. It would be like drops of water on a stone. Individually they don’t seem to alter the surface, but constant dripping wears it away. 

That being said, family studies have clearly shown that genes can account for about 50% of all differences in intelligence among people. For this type of research, comparing twins is really helpful. There are two types: (i) identical twins, who come from the same egg and sperm that divided in two at very early stages of development, and (ii) fraternal twins, originated from two different eggs and two distinct sperms. The first ones are basically clones and share 100% of their DNA. The non-identical, however, are like any other siblings and have about 50% of their genes in common. 

Scientists have found that the more DNA two people share, the more similar scores they get in intelligence tests. If we think about a famous and extensive family, let’s say the Weasleys, and we have to order them according to how comparable their IQs are, Fred and George, as identical twins, will be the most similar. The next in line will be Ron and Ginny who, as biological siblings, will have closer IQs than these of an adopted child raised in the same environment like Harry Potter. Of course, Hermione will always be the smartest person in the room, so her parents are likely intelligent too.

Does this mean that we should blame our genes for our grades? Yes… and no, a group from King’s College London revealed that there is a genetic basis for how easy or enjoyable children find learning). Yet, it does not depend just on intelligence, but also on other traits (including motivation or confidence). This means, that the current educational system favors a specific combination of attributes.

If you are among the lucky ones, you will succeed, but not having great academic achievements does not imply that you are not intelligent. A crucial discovery here is that teaching in different manners –with computer programs or with hands-on projects– could awake the drive to learn in other personality types. 

Genetics versus environment. Ready… fight!

Since I just told you that roughly 50% of the differences in intelligence can be attributed to genetics… where does the rest come from? For the other half, environmental factors are particularly important. Those include a kid’s home environment, parenting, education, availability of educational resources, or nutrition. Supporting this point, identical twins reared apart have less similar IQs than those that have grown under the same roof.

In the past, the question of whether intelligence was determined by genetics or environment confronted scientists to such an extent that fight club would look like child’s play. However, as Aristotle pointed out, virtue is found in the middle. Although it is sometimes difficult to tease them apart, there is no doubt that both affect intelligence. While genetics can determine how smart you can become, the environment influences IQ development providing opportunities or limitations. And here, we come across a serious socioeconomic problem. Whereas healthy and well-fed people have the chance to “unlock” their full potential, for the less wealthy citizens environment represents a disadvantage.

But how can our surroundings interact with our DNA? Our life experiences can determine whether a gene is actually read, or just ignored by the body. Like a switch that you can turn on and off. In theory, this is a great mechanism, as it enables our genome to adapt to our context. Yet, sustained stress or adverse life experiences, especially in our youth, may change how our genes are expressed, altering brain wiring. Dopamine, for instance, plays a crucial role in motivation. Silencing of a dopamine receptor gene by these mechanisms –called epigenetic changes– makes neurons to be less active, which has been associated with lower IQ results.

Our genome over a lifetime

An incredible fact I was keeping up my sleeve is that the genetic influence on general intelligence is not constant over our lives! Instead, it appears to increase with time. Genetic factors account for about 20% of differences in IQ during infancy, raising to 40% in adolescence and up to 60% in adulthood. 

Although we do know that intelligence changes with age, the genes associated with IQ remain constant, so how can this occur? Scientists believe that one option is that children seek experiences that correlate with their genetic propensities. Thus, we select environments in which our genetic differences snowball because we actively look for contexts that make our genes to be at ease.

When you are in primary school, you have to learn all the subjects, but as you go through school years you realize rather quickly that “I hate maths” or “I can’t stand economics” and you choose to keep studying the subjects you like the most –or dislike the least. This is an example of actively choosing our context. These decisions might very well affect epigenetics, influencing the expression pattern of our genes.

In sum, although we are born with a given genetic background that determines the upper limit of our intelligence, we can choose to actively explore our potential. We might search for challenging or comfortable environments, as each of us is inclined to different things, but unravelling our full intellectual abilities is for sure a lifetime mission. Enjoy the ride!