Over 500,000 people visited Farnam Street last month to expand their knowledge and improve their thinking. Work smarter, not harder with our free weekly newsletter which offers a collection of time-tested multidisciplinary knowledge you can add to your mental toolbox.

Our Genes and Our Behavior

“But now we are starting to show genetic influence on individual differences using DNA. DNA is a game changer; it's a lot harder to argue with DNA than it is with a twin study or an adoption study.”
— Robert Plomin


It's not controversial to say that our genetics help explain our physical traits. Tall parents will, on average, have tall children. Overweight parents will, on average, have overweight children. Irish parents have Irish looking kids. This is true to the point of banality and only a committed ignorant would dispute it.

It's slightly more controversial to talk about genes influencing behavior. For a long time, it was denied entirely. For most of the 20th century, the “experts” in human behavior had decided that “nurture” beat “nature” with a score of 100-0. Particularly influential was the child's early life — the way their parents treated them in the womb and throughout early childhood. (Thanks Freud!)

So, where are we at now?

Genes and Behavior

Developmental scientists and behavioral scientists eventually got to work with twin studies and adoption studies, which tended to show that certain traits were almost certainly heritable and not reliant on environment, thanks to the natural controlled experiments of twins separated at birth. (This eventually provided fodder for Judith Rich Harris's wonderful work on development and personality.)

All throughout, the geneticists, starting with Gregor Mendel and his peas, kept on working. As behavioral geneticist Robert Plomin explains, the genetic camp split early on. Some people wanted to understand the gene itself in detail, using very simple traits to figure it out (eye color, long or short wings, etc.) and others wanted to study the effect of genes on complex behavior, generally:

People realized these two views of genetics could come together. Nonetheless, the two worlds split apart because Mendelians became geneticists who were interested in understanding genes. They would take a convenient phenotype, a dependent measure, like eye color in flies, just something that was easy to measure. They weren't interested in the measure, they were interested in how genes work. They wanted a simple way of seeing how genes work.

By contrast, the geneticists studying complex traits—the Galtonians—became quantitative geneticists. They were interested in agricultural traits or human traits, like cardiovascular disease or reading ability, and would use genetics only insofar as it helped them understand that trait. They were behavior centered, while the molecular geneticists were gene centered. The molecular geneticists wanted to know everything about how a gene worked. For almost a century these two worlds of genetics diverged.

Eventually, the two began to converge. One camp (the gene people) figured out that once we could sequence the genome, they might be able to understand more complicated behavior by looking directly at genes in specific people with unique DNA, and contrasting them against one another.

The reason why this whole gene-behavior game is hard is because, as Plomin makes clear, complex traits like intelligence are not like eye color. There's no “smart gene” — it comes from the interaction of thousands of different genes and can occur in a variety of combinations. Basic Mendel-style counting (the sort of dominant/recessive eye color gene thing you learned in high school biology) doesn't work in analyzing the influence of genes on complex traits:

The word gene wasn't invented until 1903. Mendel did his work in the mid-19th century. In the early 1900s, when Mendel was rediscovered, people finally realized the impact of what he did, which was to show the laws of inheritance of a single gene. At that time, these Mendelians went around looking for Mendelian 3:1 segregation ratios, which was the essence of what Mendel showed, that inheritance was discreet. Most of the socially, behaviorally, or agriculturally important traits aren't either/or traits, like a single-gene disorder. Huntington's disease, for example, is a single-gene dominant disorder, which means that if you have that mutant form of the Huntington's gene, you will have Huntington's disease. It's necessary and sufficient. But that's not the way complex traits work.

The importance of genetics is hard to understate, but until the right technology came along, we could only observe it indirectly. A study might have shown that 50% of the variance in cognitive ability was due to genetics, but we had no idea which specific genes, in which combinations, actually produced smarter people.

But the Moore's law style improvement in genetic testing means that we can cheaply and effectively map out entire genomes for a very low cost. And with that, the geneticists have a lot of data to work with, a lot of correlations to begin sussing out. The good thing about finding strong correlations between genes and human traits is that we know which one is causative: The gene! Obviously, your reading ability doesn't cause you to have certain DNA; it must be the other way around. So “Big Data” style screening is extremely useful, once we get a little better at it.


The problem is that, so far, the successes have been a bit minimal. There are millions of “ATCG” base pairs to check on.  As Plomin points out, we can only pinpoint about 20% of the specific genetic influence for something simple like height, which we know is about 90% heritable. Complex traits like schizophrenia are going to take a lot of work:

We've got to be able to figure out where the so-called missing heritability is, that is, the gap between the DNA variants that we are able to identify and the estimates we have from twin and adoption studies. For example, height is about 90 percent heritable, meaning, of the differences between people in height, about 90 percent of those differences can be explained by genetic differences. With genome-wide association studies, we can account for 20 percent of the variance of height, or a quarter of the heritability of height. That's still a lot of missing heritability, but 20 percent of the variance is impressive.

With schizophrenia, for example, people say they can explain 15 percent of the genetic liability. The jury is still out on how that translates into the real world. What you want to be able to do is get this polygenic score for schizophrenia that would allow you to look at the entire population and predict who's going to become schizophrenic. That's tricky because the studies are case-control studies based on extreme, well-diagnosed schizophrenics, versus clean controls who have no known psychopathology. We'll know soon how this polygenic score translates to predicting who will become schizophrenic or not.

It brings up an interesting question that gets us back to the beginning of the piece: If we know that genetics have an influence on some complex behavioral traits (and we do), and we can with the continuing progress of science and technology, sequence a baby's genome and predict to a certain extent their reading level, facility with math, facility with social interaction, etc., do we do it?

Well, we can't until we get a general recognition that genes do indeed influence behavior and do have predictive power as far as how children perform. So far, the track record on getting educators to see that it's all quite real is pretty bad. Like the Freudians before, there's a resistance to the “nature” aspect of the debate, probably influenced by some strong ideologies:

If you look at the books and the training that teachers get, genetics doesn't get a look-in. Yet if you ask teachers, as I've done, about why they think children are so different in their ability to learn to read, and they know that genetics is important. When it comes to governments and educational policymakers, the knee-jerk reaction is that if kids aren't doing well, you blame the teachers and the schools; if that doesn't work, you blame the parents; if that doesn't work, you blame the kids because they're just not trying hard enough. An important message for genetics is that you've got to recognize that children are different in their ability to learn. We need to respect those differences because they're genetic. Not that we can’t do anything about it.

It's like obesity. The NHS is thinking about charging people to be fat because, like smoking, they say it's your fault. Weight is not as heritable as height, but it's highly heritable. Maybe 60 percent of the differences in weight are heritable. That doesn't mean you can't do anything about it. If you stop eating, you won't gain weight, but given the normal life in a fast-food culture, with our Stone Age brains that want to eat fat and sugar, it's much harder for some people.

We need to respect the fact that genetic differences are important, not just for body mass index and weight, but also for things like reading disability. I know personally how difficult it is for some children to learn to read. Genetics suggests that we need to have more recognition that children differ genetically, and to respect those differences. My grandson, for example, had a great deal of difficulty learning to read. His parents put a lot of energy into helping him learn to read. We also have a granddaughter who taught herself to read. Both of them now are not just learning to read but reading to learn.

Genetic influence is just influence; it's not deterministic like a single gene. At government levels—I've consulted with the Department for Education—I don't think they're as hostile to genetics as I had feared, they're just ignorant of it. Education just doesn't consider genetics, whereas teachers on the ground can't ignore it. I never get static from them because they know that these children are different when they start. Some just go off on very steep trajectories, while others struggle all the way along the line. When the government sees that, they tend to blame the teachers, the schools, or the parents, or the kids. The teachers know. They're not ignoring this one child. If anything, they're putting more energy into that child.

It's frustrating for Plomin because he knows that eventually DNA mapping will get good enough that real, and helpful, predictions will be possible. We'll be able to target kids early enough to make real differences — earlier than problems actually manifest — and hopefully change the course of their lives for the better. But so far, no dice.

Education is the last backwater of anti-genetic thinking. It's not even anti-genetic. It's as if genetics doesn't even exist. I want to get people in education talking about genetics because the evidence for genetic influence is overwhelming. The things that interest them—learning abilities, cognitive abilities, behavior problems in childhood—are the most heritable things in the behavioral domain. Yet it's like Alice in Wonderland. You go to educational conferences and it's as if genetics does not exist.

I'm wondering about where the DNA revolution will take us. If we are explaining 10 percent of the variance of GCSE scores with a DNA chip, it becomes real. People will begin to use it. It's important that we begin to have this conversation. I'm frustrated at having so little success in convincing people in education of the possibility of genetic influence. It is ignorance as much as it is antagonism.

Here's one call for more reality recognition.


Still Interested? Check out a book by John Brookman of Edge.org with a curated collection of articles published on genetics.