Is Most of Your Intelligence Decided by Your Genes?

If every you see people or media talk about IQ tests, you might have heard that something as astonishing as 80% of your IQ is decided by your genes. Is that really true?

The short answer is no. I intentionally phrased it wrong to lead you astray. So let us clear up some things.

Hereditary vs Genetic

These two concepts are often used as if they are the same. Here is a simple way to understand the difference. You can get a mutation in a gene in your DNA which causes say cancer. That means that type of cancer is genetic. It is something in your DNA which is causing it. However it is not something hereditary. A trait is hereditary if it is passed on from your parents. Eye color or hair type would be both genetic and hereditary. It is genetic because it comes from your genes and it is hereditary because it got passed from your parents.

So here is a funny thing people often aren’t aware of which is that a trait could be hereditary but not genetic. An example would be the language you speak. A lot of habits and customs get passed on through generations.

It is useful to note however that based on my own perception of the usage of the words, it seems most of the time when researchers doing genetics speak of hereditary treats they are only talking about genetic ones.

Variation Not Portion

The correct way to talk about IQ and heritability is not about how much of my IQ can be attributed to nature or nurture, but rather how much of the variation in IQ found in a population studied in a particular environment is caused by heritable traits. That is when studying a population, to what degree can I predict people’s IQ by simply looking at the IQ of their parents.

To exclude the effects of the environmental influence of parents, we typically do twin studies as well as look at adopted children.

The Illusion of Genetic Determinism

A problem when discussing this is that it has a tendency to cause a genetic determinism in people’s political views. People think that a very high heritability of IQ, means the genes you get at birth will determine your whole future and there is little you can do about it.

It fuels some dangerous political ideas, where populations where IQs are measured to have a lower average and viewed as inferior and a danger to ones society. There is this sense that “inferior” people may pollute the gene pool and lower the average IQ.

This is part of the reason why I am writing about this, because the relationship between genes, intelligence, the environment and different human populations is so poorly understood by people.

Intelligence is such an abstract concept that it is hard to get a good intuition about how it related to the environment. It is much easier to understand if you consider intelligence to be much the same as any other human trait such as strength, weight and height.

Discovery Magazine has a discussion of height and weight, where it is stated:

The heritability of weight is between .75 and .85. The heritability of height is between .9 and .95. And the older you are, the more heritable weight is.

This means that e.g. 90% to 95% of the variation of height you see in a population can be explained by genetic factors. However this easily gives people the misleading notion that whatever difference we see between various human populations must primarily be cause by genetic differences.

Except this can’t be true, because the relative difference between various human populations has changed significantly through history. The dutch used to be among the shortest in Europe about 150 years ago. Today they are the tallest in the world. White Americans used to be almost a head taller than white Europeans and now Europeans have surpassed Americans in average height.

The Japanese used to be considerably shorter than Americans, but have now nearly caught up with Americans in average height. Clearly nutrition and living conditions have a major impact on human height.

So how can the measured heritability of height be so high? You can think about this with a thought experiment. Have five plots of land of different soil quality. On each plot of land plant ten different seeds which are genetically different from each other. However for each plot of land we are using the same ten seeds (say we clone them).

The average height of the plants for each plot of land will be proportional to the soil quality of that plot of land. However the internal variation of height within one plot of land, will be determined almost exclusively by genetics.

The same happens for human populations. One could say that people in different countries are planted in soil of different quality. The quality of the environment people grow up in will vary. The food, health care, pollution etc. These difference will determine most of the average differences found between populations.

However within one environment, variations can be explained mostly by genetic correlations.

Minorities and the Environment

It is important to not however that one country does not represent one homogenous environment for all inhabitants. Every country contains sub-environments of different quality. In the 1800s e.g. the British upper class was a whole head taller than the British lower classes.

Today we can find minorities of lower socio-economic class being either shorter or significantly more overweight than the majority population. Or there could be minorities with significantly better outcomes.

One can’t use such observations to conclude that somehow these other sub populations of people are somehow inferior, because you are doing an apples to oranges comparison if their environments are not comparable.

If one group of people has poorer access to education, nutrition, work, safe neighborhoods etc then that will create an environment which will have a lower natural average for e.g. intelligence.

The Complex Interaction Between Nature and Nurture

To further hammer this point how it is worth elaborating on another common misunderstanding which is to think genes and environment sort of work independent from each other.

Say we discover genes for strong muscles. It is easy to think this means there are genes which cause your muscles to naturally grow bigger than your peers. Say two kids are raised identical, then the one with the muscle genes would be assumed to get bigger muscles.

That could be the case, but that is not always the case. Rather muscle genes could cause their effect in a wide variety of ways. It could be genes which really just affect your interests, so that somebody having the muscle genes simply have more interest in working out and lifting heavy things. Hence it is by working out more they get bigger muscles.

An alternative effect could be that there isn’t a bigger interest but one recuperates faster after lifting weights, hence enabling more frequent workouts.

Vocabulary is has a strong heritable component. But of course words are not encoded in your genes. Children have to interact with the environment to acquire new words. Genes for high vocabulary hence is more about affecting e.g. interest in learning new words or ability to remember taught words. However clearly if there is no environment where a grownup is there to teach words, these genes are not going to help.

Hence a child with talkative parents could gain a higher vocabulary than one with mostly silent parents but with better genes for vocabulary.

This also gives some clues as to why heritability tends to increase with age. IQ has a heritability of 50% with children but increases up to 80% with adults. A child curious of new words could get hampered by parents, but as time passes and they get older they get increased opportunities to select environments themselves where there is an abundance of words. Just as say a curious person could increase their IQ over time by being able to select environments which stimulate their learning and mental challenges.

If you are around children it is not hard to see how this plays out. The ability of a child to concentrate on a task can vary quite a lot. Two children may initially be equally good at drawing or solving math problems. However the child who is able to sit concentrate working through problems for longer periods will in time outperform their peer.

The same can be observed with sports. The kids who get really good are often relentlessly trying and testing different things, trying to get better. They might not be able to stop doing it because they think it is so much fun. There might not be any initial difference between two kids, in their ability to kick a ball around. But the kid who are given genes which make the child enjoy kicking a ball around more than other kids, will eventually get significantly better.

Compound Interest

Like compound interest small differences in talents can accumulate to dramatic differences over time. A slight preference to perform a particular task, can result in a large difference in ability over time. One start example of this was presented by Malcom Gladwell in Outliers. Statistics showed something remarkable, which was that almost all the top Ice-hockey players in Canada were born in the first months of the year. The explanation was that at age 8, when Canadian hockey coaches start streaming the best players, the older ones (that is those born earlier in the year) will have an advantage. These kids get picked to play more games, get more attention etc, which improves their skills slightly furthering their advantage over kids born later in the year. Better skills lead to more playing and better follow-ups which like compound interest grows over time to become quite dramatic.

This is something we should always keep in mind when comparing intelligence or talents between people from different populations. Do we in fact have a system that amplifies differences that might not be that big to begin with or that reduces differences?

Think about how this can play out for IQ. A slight advantage could initially be had from simply being born in the right neighborhood and go to the right school. Compared to peers going to worse schools, this can translate to enough advantage to land you a scholarship, which gets you into a better school than your peers hence further amplifying your advantage.

It doesn’t stop there. This difference can get amplified over generations. As compound interest of success grows, you end up with more opportunities, a better job, better pay and eventually a better neighborhood. That means your children will now get an extra advantage. They live in a more nurturing environment with other accomplished kids and good parents. Your kid goes to a better school.

Meanwhile the kids that slip behind have to raise their kids in perhaps a bad neighborhood. The school isn’t any good and there are distractions like criminal gangs, lowering potential. As a parent you might not have time to really raise your kid well. You work 3 jobs to pay the rent and put food on the table. There is no time for taking your kids to museums or extra curricular activities. Your kid falls behind and never gets that scholarship. Your child and the child of your peer who started childhood with a slight advantage has suddenly turn into a significant difference.

Intelligence is not all that different from a muscle. You need to work out and build it continuously. If you don’t work out for a long time, you start falling behind those who do.

Breaking the Circle

One example of how society must not fall into the trap of worshipping genetic determinism is from my home country Norway. Now I don’t follow cross country skiing very much, but as Norwegian it is hard to not know something about it. Marit Bjørgen and Therese Johaug are among the best, if not the best cross country skier in the world.

They are great examples of how a genetic advantage might not be in terms of raw talent or muscle but rather in the interaction with personal motivation and the environment.

Therese Johaug was not a talented skier when she was young. In fact her sister was a much bigger talent. Yet her sister never made it too the Olympics while Johaug won the gold medal. Interest, drive and persistence was the key difference. Johaug kept trying to get better while her sister did not pursue the sport further.

Marit Bjørgen was dismissed repeatedly as a young skier and told that she simply did not have a body that was required to become a top cross-country skier. Bjørgen had more the build of a sprinter, which meant she did not have the stamina for long range skiing. Yet like Johaug, she never gave up and worked at overcoming her physical disadvantages while utilizing her unique strengths. E.g. she uses ski poles more active than her competitors utilizing the fact that she has much more upper body strength than what is normal for a skier.

The key thing was that neither Johaug, nor Bjørgen was prevented from pursuing their interest even if they were no obvious natural talent initially.

Likewise in school there might be kids who are at a disadvantage initially but if given they chance could find a way to overcome their disadvantage. Unfortunately many never get that chance. They lose that scholarship or don’t dear take further schooling because it will set back their parents too much financially.

That is my take on how to approach minorities. If they look worse than everybody else, don’t use that as an excuse to give up on them and deprive them of chances. Instead look for ways in which one can help them overcome their disadvantages. Perhaps they have other strengths they can use to compensate.

Don’t let the logic of compound interest let a minor disadvantage turn into a major disadvantage over time.

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