Is intelligence determined by genetics?

What’s more important – nature or nurture?

Is intelligence determined by genetics? Is it a product of what you learn when you’re growing up?

Or does it come from a combination of both?

Like many characteristics, intelligence seems to have a high genetic influence, but it’s a whole lot more complicated than just that.

Even defining the concept of intelligence and measuring it is so difficult that we have to add a lot of other information into the mix.

So, while large, reliable studies have shown that genetics most definitely play a role in intelligence, that’s by no means the end of the story.

What is intelligence?

If we’re going to talk about what produces or influences intelligence, we should first have a very good idea of what intelligence is.

So, what does “intelligence” mean?

Most of us have a very clear idea in our minds of what an intelligent person is like. 

They can do math in their heads. They’re good and puzzles. They can remember obscure facts from history. They can come up with creative solutions to the problems they encounter.

However, the problem is that my idea of smarts and yours could be wildly different.

That’s why experts need to share a standard definition of intelligence in order to study it and compare tests. But the problem is that there is really no consensus in the scientific community.

For the most part, different researchers define intelligence in their own ways. However, most agree on the following constructs:

Fluid intelligence

Frequently, their definitions include an ability to learn from experiences and adapt to changes in one’s surroundings. 

This can be considered fluid intelligence and is measured by a person’s ability to solve abstract reasoning problems like pair associations and number series that a person has never seen before. 

This part of a person’s intelligence is called “fluid” because the person’s skills are able to flow into new situations and problems without having to pre-learn anything.

Crystallized intelligence

Experts also use the concept of crystallized intelligence, which includes the facts and procedures that a person has learned. 

This part of intelligence is measured by testing things like vocabulary and mechanics of language as well as fact-checking. 

Crystalized intelligence is influenced by a person’s accumulation of skills and knowledge and can, therefore, grow throughout their lifetime as they learn and memorize more.

General intelligence

Together, fluid and crystallized intelligence make up the g factor or a person’s general intelligence.

G includes a person’s ability to reason, solve problems, think in the abstract, plan, and comprehend difficult and complex concepts.

Tests to determine a person’s level of general intelligence are usually called IQ tests.

Measures of intelligence

When we want to find out how smart someone is, we normally think about tests and performance in academic situations.

IQ tests

Intelligence quotient tests are designed to measure a person’s intelligence level compared to that of the general population. So, IQ tests are standardized to adapt the mean score to 100. 

This means that no matter how many questions or points are given on the test, the average is set to 100. Anyone who tests below average will get a score of less than 100, while people testing above average are given scores over 100.

IQ tests are quite reliable, meaning that people who test again and again will receive very similar scores.

However, they’re also criticized for focusing on academic intelligence and ignoring other skills and abilities.

School grades

Another simple measure of intelligence we’re all familiar with is the grades you get in school.

While grades can reflect an overall ranking of student performance, they’re not extremely reliable at measuring actual intelligence.

And it’s easy to see why.

First, a very bright student might struggle in one particular subject, like French, for example. While they do well in all their other subjects, they may not be skilled in foreign languages, so their score in French might not accurately represent their intelligence.

Second, one bright student might not work hard in school and would get mediocre grades. Another student of lesser intelligence may apply themself more and receive the same mediocre grades. 

On paper, these students would seem to have equal intelligence.

Third, different schools and even different teachers give grades according to different criteria, making them hard to compare across different institutions. Is an ‘A’ grade from Harvard equal to an ‘A’ grade from a community college in Liberia?

Overall, it’s very difficult to measure intelligence, and researchers simply have to do the best they can with the tools they’ve developed.

Intelligence correlations

signs youre not as intelligent as you think you are Is intelligence determined by genetics?

Some interesting patterns emerge from IQ test scores no matter which tests are taken. These patterns suggest that even though a standard definition of intelligence eludes researchers, they’re on the right track.

Correlations show how two factors are related and are measured from zero to one. 

A correlation of zero means that the two things are completely unrelated, while a score of one means they are 100% related (like being a human and having skin, for example).

Some interesting things have been found to be correlated with IQ scores.

There is a small correlation of 0.31 between income and IQ, meaning that having a higher IQ contributes somewhat to making more money.

There is a quite high correlation of 0.55 between achieved level of education and IQ, meaning that people with higher IQs tend to study to higher levels and attain more degrees.

Conversely, there is a negative correlation between crime and an IQ of -0.20, meaning that people with higher IQs tend to commit slightly less crime than people with lower IQs.

Job success, better health, and academic success are all correlated with higher intelligence.

So, can all this be predicted by genetics?

Heritability of intelligence

In the field of genetics, experts use the term “heritability” to describe how much variation in a trait is based on genes.

Suppose most people have IQs in the range of 80-120. If genetics was the only cause of variation, then we’d say that this range was 100%. That is, 100% of the reason why people have different IQ test scores is because of their genes.

But as it happens, nothing is 100% or 0% heritable.

In the case of intelligence, very large and wide-ranging studies have found that intelligence is anywhere from 50% to 80% heritable.

How is that proven?

The best way to prove genetic factors is through twin studies, especially of identical twins who are separated and raised in different environments, like if their parents divorce or if they’re adopted and go to separate homes.

Since twins essentially share all the same DNA, it’s like studying a single person in two different environments.

So what these studies show is that even though they live in different places and with different people, twins have a much higher chance of having similar IQ scores than any two random people would have.

That means genes must play a role in intelligence!

At the same time, no specific “intelligence genes” have yet been isolated, so researchers can’t be specific about which shared genes will cause people to inherit intelligence from their parents.

Another interesting finding comes from academic achievement studies.

In one major 2014 study, it was found that 62% of the variability in academic performance came from genes.

This study looked at the influence of motivation, personality, confidence, and a dozen other factors, including IQ. It found that twins who had similarities in these other factors also had more similar academic performance.

So, how well you do in school is also highly controlled by your genes, in much the same way that IQ is.

Heritability and age

If it’s very tricky to measure intelligence in adults, imagine how hard it is in infants and children!

And yet, researchers have found ways to test intelligence even for the youngest children and have found that the heritability of intelligence changes over time.

Heritability in infants has been found to be just 20%, and in children, it goes up to 40%. In adults, it can reach anywhere from 50 to 80%.


This suggests that the genes that influence intelligence have an amplified influence over time. They could be triggered by age or continued use so that they end up having more and more of an effect.

What else determines intelligence?

If genes control 50-80% of variation in intelligence between people, what accounts for the other 20-50%?

The answer has to be the environment and the experiences of each person living their life.

Nutrition, socioeconomic status, environmental pollution, parental involvement, quality of education, and many other factors can combine to influence intelligence.

So is intelligence determined by genetics?

Studies suggest that a good part of our intelligence does get inherited, but not all of it.

If you have brilliant parents, but you’re raised in a stressful war zone with little to eat, you may not gain all of their smarts.

But even if your parents are a bit dim but you have the best education money can buy, you might end up being super-smart.

Marcel Deer

Marcel Deer

Marcel is a journalist, gamer, and entrepreneur. When not obsessing over his man cave or the latest tech, he’s failing helplessly at training his obnoxious rescue dog ‘Boogies’.

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