The genetics of intelligence: What we know about the heritability of IQ

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Scientists now know that a large amount of intelligence is inherited and that our genes play a crucial and central role in our IQ. 

But how does this process work and just how far does the influence of DNA go in determining our IQ? 

Here’s what we know about the heritability of IQ. 

How genes work

First, it’s crucial to understand the basics of how genes work and how we inherit intelligence.

At birth we get all our genes from our two parents. 

Our mother has two genes for eye color (from her two parents) and our father has two genes for eye color (from his two parents). 

They randomly pass on one of these two genes to us to comprise each of the 20,000 or so genes in our body. 

For example, taking the eye color scenario. 

Certain genes are recessive and don’t appear, while others are dominant and will “win out” over the other gene. 

For example, if your dad has brown eyes and your mother has blue eyes, you will end up with brown eyes, since brown eye genes are dominant over blue eye genes. 

But your brother from the two same parents may end up with green eyes, because your father had one set of brown eye genes, one set of green eye genes and your mother had one set of blue eye genes and one set of green eye genes. 

Each time there is reproduction there is an allotment of these genes, and the same goes for intelligence.

When two parents have kids, their intellectual, verbal and mental traits are passed down in varying and very complex forms to their kids.

This has a massive impact on the future of the children being born, because their DNA won’t just determine how they look and talk, it will also have a big stake in determining how intelligent they are and their personality

It won’t determine everything about how smart they are, but it will determine a lot. 

And it all goes back to the DNA, that sequences of genes imparted to us from our biological parents. 

DNA is our blueprint

Deoxyribonucleic Acid (DNA) is the blueprint for who we are as a human being. 

DNA is inherited from both parents and sends instructions and orders that have a formative influence on our physical appearance, our intelligence, our sense of humor, our mental health and everything else about us. 

As Professor Robert Plomin notes in the Scientific American

“The DNA differences inherited from our parents at the moment of conception are the consistent, lifelong source of psychological individuality, the blueprint that makes us who we are.”

While this DNA is certainly not a “finished three-dimensional” structure and the environment can certainly alter it, our “genetic trajectory” will always reassert itself over and above the vast majority of outer influences. 

When it comes to our intelligence, DNA also serves as our primary blueprint, defining a lot of the capacity and mental flexibility we are able to put to use. 

No matter what family background we are raised in, what education we are given and what advantages or disadvantages we are handed in life, a large part of how smart we will be is already determined at birth. 

The two intelligences and capabilities of our parents form together and produce us: a unique individual with a partly predefined capacity to be intelligent. 

However, the environment and its impact on our intelligence certainly does matter, just not quite in the way we think. 

Genes matter a lot, but they’re not written in stone

The current scientific consensus is that IQ is 50% to 80% determined by genes

In other words, at least half of how intellectually smart you are or will ever be is already contained in your DNA. 

Various peer-reviewed research done with identical twins who grew up in different environments has been used to come up with these conclusions. 

The variation represents the vast majority of scientific thought on the subject, with some identifying genes as responsible for as little as half of IQ and others identifying genes as responsible for up to 80% of adult IQ.

Any researcher, however, will agree that heritability of IQ is only part of the story and that the environment has a big influence as well. 

The environment has at least a 20% influence on your IQ and up to a 50% influence. 

Especially in early childhood, intellectual and verbal intelligence are greatly influenced by the environment.

This triggers the epigenome, or epigenetics, which are parts of the DNA that don’t always manifest and depend on environmental stimuli. 

The environment includes influences such as: 

  • Stability of the pregnancy itself and fetal health
  • Nutrition, water quality, pollution
  • Education level, culture, family literacy

These kinds of factors have a huge influence on a growing child, but there’s a twist about why that is. 

Indeed, in the debate of nature versus nurture, the majority of scientific literature comes down on the side of nature. 

In fact, there’s a big twist about the environment that is revolutionizing the scientific field. 

Why nature ultimately wins out over nurture 

The reason that nature ultimately wins out over nurture is that nurture is not as measurable or linear an influence as scientists once thought. 

Whereas Freudian psychology and other analysts once thought that our family background is clearly our major formative influence, science has shown that who we become is actually very difficult to measurably trace to our family influences. 

It’s much easier to trace to our genetics and inherited characteristics and talents that we were born with, because a definite through-line exists. 

As Plomin notes

“One of the most remarkable discoveries is that even most measures of the environment that are used in psychology—such as the quality of parenting, social support and life events—show significant genetic impact.”

This is huge, if you understand the gravity of what Plomin is suggesting here, which is that the environment itself is orbiting around a genetic constellation. 

As Plomin explains, our environment itself is partly a product and reflection of genetics and genetic selection. 

“Genetic influence slips in because the environment is not randomly ‘out there’ independent of us and our behavior. 

We select, modify and even create our environments in line with our genetic propensities.”

Plomin serves as professor of behavioral genetics at the Institute of Psychiatry, Psychology & Neuroscience at King’s College London and he most certainly knows what he’s talking about far more than most. 

A typical example of the nature embedded in nurture can be found in breastfeeding:

Yes it has a big impact on an infant’s later personality and development in various ways, but this mostly depends on the already existing genes of the baby being breastfed.

Taking a deeper look at nature

When we take a deeper look at nature, we find that it is also something of a mystery.

After all, what were the original factors that led to any two people being more intelligent than any two others if not environmental factors and adapting to survive to the outer environment?

It would seem, by the Darwinian model, that intelligence embedded itself in genes as part of survival traits and tried to pack as much preset information, reflexes and capacities into the starting software as possible. 

Noam Chomsky, for example, convincingly demonstrated his theory for why language learning is an innate mental capacity held from the moment of birth, and is not “learned” or created. 

So much of what we are is absolute and inherent, and not about the software that our culture or tribe installs into us. 

While environments bring out or inhibit various parts of that software, no convincing case has been made for the environment acting as the prime mover of a person’s intelligence

The science is clear

Past thinkers like John Locke believed in tabula rasa, that we are a blank slate shaped entirely by our experiences and environment. 

Then science progressed and thoroughly disproved this thesis.

But the popularity of emphasizing outer influences has continued in various social sciences. 

It’s popular to say that nature and nurture are like a yin-yang relationship working together and that neither outweighs the other

But that’s not really true. 

While our environment clearly has a big influence on IQ in unpredictable ways, it’s not the giant factor it was once built up to be.

This is especially key to absorb, since the environment is partly a reflection and selection formed by genetic traits. 

At this point we know that the environment and outer influences have a somewhat erratic and unpredictable influence, while nature has a steady, heritable hand in deciding how smart you will be. 

The jury is in: 

Nature far outweighs nurture. 

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