Why are my children so smart?

May 26, 2006

A curious adult from Alabama asks:

"Why are my children so smart?"

They are so smart because of the genes their parents gave them. And the environment they were raised in. Nicely done!

But how much is because of how you raised them? And how much is due to how their brains are wired? These sorts of debates have been going on for at least 50 years. Scientists call it the “nature versus nurture” controversy.

As you might imagine, it is really hard to figure out which is most important for intelligence. First off, we need to be smart enough to figure out what intelligence is so we can measure it.

For this discussion, intelligence is the ability to learn and then apply new facts and concepts. A good example is learning to recognize clumps of letters as words to learn to read. Or combining numbers to do math.

Of course this isn't all of intelligence. This is just a form that is easy to measure.

We know genes can be involved because there are genetic conditions that affect intelligence. Some like Fragile X are the result of a single gene. Others like Down syndrome are the result of an extra chromosome 21. So genes are important for intelligence.

But so is the environment. Think about the effects of a poor diet. Or an overcrowded school, abusive parents, etc. All of these will clearly affect any tests that measure intelligence.

OK, so genes and the environment are important for intelligence. Is there any way to figure out which matters more? It is tricky, but there are some good studies out there.

Child in front of chalkboard
We don’t fully understand how intelligence works, but we know that both genes and the environment are involved. (Image via Shutterstock)

Twin studies of intelligence

Since each person is unique, scientists had to be very creative when designing experiments to look at intelligence. In the end, they came up with adoption studies of identical twins. Here's how it works.

Identical twins share the same DNA. This means that for the most part, their genes are the same. So any differences in intelligence would be due to the environment.

But the environments of identical twins are actually pretty similar. They grow up in the same house at the same time, so it can be very hard to tease out environmental differences.

To get around this, scientists look at identical twins who were raised apart.1 These are people who share the same DNA but grow up in different environments.

So what did scientists learn from these studies? First off, the IQs of the twins were closer to their biological parents than they were to their foster parents. So genes matter.

But they didn't have identical IQs. So the environment plays a role too. But how much of a role does each play in intelligence?

Well, it's about 50% each. But if we look a little closer, the relative importance of each changes as we age. Environmental factors are most important for infants. Genes matter most to adults.

If you think about it, this makes sense. During infancy, our brains are still growing and developing. And the environment can certainly influence that!

The environment affects how our brains get wired. A poor environment leads to fewer and/or weaker connections. And a good environment lays down a solid foundation of wiring.

Twin girls with books
Scientists have learned a lot about intelligence by studying identical twins, especially ones who were raised in different households. (Image via Shutterstock)

Genes and intelligence

Genes can affect the wiring too. But as you can imagine, with around a trillion cells in the brain and 100 billion connections, lots of genes are involved.

These genes direct where and how the wiring is laid down and maintained. They also determine what signals are sent and how strong they are. They help determine which parts of the brain will connect to each other. And lots of other things too.

Let's look at one gene scientists found called dysbindin. This gene is involved in helping different parts of the brain talk to each other. And in maintaining these contacts.

Each of us has our own version of this gene, and some versions work better than others. The better the version, the smarter you may be.2 But not by a whole lot.

As you'd expect, dysbindin accounts for only a small part of intelligence, around 3% or so. Lots of other genes are doing similar things and we all have different versions of each of these genes too.

So the part of our intelligence that is genetic is determined by all of these different gene versions working together. The environment then helps or hinders these genes in doing their jobs. Our final level of intelligence is a combination of all of these genes and the environment!

So stay tuned. Scientists are working hard to learn more about intelligence! We'll need someone smart like your kids to help figure this all out.

Author: Dr. David Skibbe

When this answer was published in 2006, David was a postdoctoral fellow in the Department of Biology, studying maize molecular genetics in Virginia Walbot's laboratory. David wrote this answer while participating in the Stanford at The Tech program.

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