Linking Human Behavior and Genes

By Robert Hazen, George Mason University

Molecular biologist Torsten Wiesel said: “Perhaps most disturbing to our sense of being free individuals, capable to a large degree of shaping our character and minds, is the idea that our behavior, mental abilities, and mental health can be determined or destroyed by a segment of DNA.” Like it or not, there exist incontrovertible links between human behavior and genes.

Role of Genes

The links between human behavior and genes were discovered essentially by accident, in studies of drugs, in studies of illness, and other seemingly non-behavioral phenomena.

For example, there’s a defective gene that blocks hunger suppression; which can lead to overeating. We’ve perhaps known a child that has Down syndrome—that’s a genetic disease, but these children are exceptionally kind and gentle. On the other hand, another genetic disease, Tourette’s syndrome, may lead to people who display extreme antisocial behavior. In the case of these genetic diseases, there’s a clear link between a gene and a behavior.

Equally important is recognition of the familiar concept of ‘one gene, one disorder’, that defects in single genes are necessary and sufficient to cause specific diseases, but we can’t extend that to behavioral traits. Behavioral traits are much more complicated; they are the result, at best, of a large concatenation of many genes, as well as environmental characteristics. There’s not a single gene for people who are happy, or a single gene for a musician, or single crime genes, certainly. Rather, there is complex behavior and abilities that arise from this interaction of a product of a whole collection of genes and many environmental factors.

This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.

Studying Behavior in Animals

There are daunting ethical problems in studying human behavior, and so, at least for the time, animals provide an ideal means to study behavior without raising many of those difficult social and political questions that are associated with human behavioral research.

We have centuries of observations on selective breeding of domestic animals, and these demonstrate beyond any reasonable doubt that behavior is determined in part by heredity. Pioneering studies began in the 1960s to understand some of these genetic relationships to behavior.

In one widely used experimental protocol, biologists crossbred two strains of animals that displayed contrasting behavior. Pure-bred, highly aggressive mice were crossed with docile mice, for example. There was a team of British and American scientists, led by the behavioral geneticist Jonathan Flint, who basically used this protocol to identify DNA segments that are found primarily in unusually fearful mice.

The ’Knockout Mice’

This is an amazing strategy; we have to decide how aggressive or fearful a mouse is, and this is how we do it. Since mice display a wide range of behavior, we put a mouse into a brightly lit, open area. We’ll see that some mice actively explore their new surroundings, running all over, just sniffing around, while others cower in one corner “while defecating copiously”. Those are the fearful mice. We can measure this wide range of behavior, and give a scale, perhaps of 1 to 10, of a qualitative estimate of how aggressive or fearful a particular mouse happens to be. What Flint and co-workers found was a high degree of correlation between fearfulness and three specific DNA segments.

Then we have studies of what are called ‘knockout mice’. These are mice in which researchers specifically inactivate a gene, a gene that then can reveal a clear link between mouse behavior and that particular gene.

Extreme Aggression

In 1993, a team of scientists led by the French microbiologist Rene Hen targeted a gene that activates specific neuroreceptors for the chemical serotonin (this transmits signals in both mice and in human beings). These knockout mice displayed extreme aggression, leading the French scientists to speculate that similar defects might contribute to abnormal human behavior, extreme aggression in humans.

Human subjects are vastly more difficult to study than mice, and probably for a time, any controversial conclusions about heredity and human behavior are going to be the subject of scathing attacks on grounds of questionable experimental methods, such as sample size or lack of sufficiently rigorous controls.

Availability of Genetic Information

But let us imagine a time when every genetic predisposition of every individual can be known. When we have aspects of health, personality, intelligence, and behavior, and all can be quantified before birth, because we can extract a single cell from an embryo, we can do a complete genome on it, and we already have all of these tests where we know the characteristics that lead to various behaviors, various abilities, various potential medical problems. Who should have access to that genetic information?

We’ve come to a point in human history where humans can identify any individual from his or her unique genome, the genetic material of a single cell. Using the polymerase chain reaction, or PCR, we can take DNA from that single cell and amplify it, and analyze it, and this information can be used in a whole variety of ways. We can use it for DNA fingerprinting, for identifying criminals or victims from the DNA of a single cell, or groups of cells. We can use it for genetic testing, perhaps looking for specific markers for diseases, or other genetic traits in other markers.

Now we have discoveries in the field of behavioral genetics, and we someday may be able to predict the character traits, the abilities, potentially the flaws of children before they’re even born, and as a society we have to make a decision on how we’re going to use that information.

Common Questions about Linking Human Behavior and Genes

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