How Much Do Genes Affect Human Behavior? [Genes & Behaviors: I]
Colloquially, genes influence to a significant extent how humans behave. Obesity is said to be caused by one’s genes, aggressive behaviors have genetic underpinnings, and if someone happens to become a criminal, it was an inevitable outcome given their genetic makeup.
When mental disorders are regarded as biological diseases, a sufferer is deemed less responsible for their actions.
That same person is now viewed in a more negative light as incurably dangerous, leading the sufferer to feel less in control of the illness and thus more likely to fall into depression.
In a nationally representative survey of American adults, 76 percent believed that “single genes directly control specific human behaviors.”
A British survey showed that people support the idea that the police should take DNA samples from individuals charged with a crime, where the seriousness of the crime affected the amount of support:
- Shoplifting: 36 percent,
- Drunk-driving: 56 percent.
- Murder: 98 percent.
Furthermore, the respondents with lower levels of knowledge about genetics (and/or religious beliefs affecting their decisions) were more likely to consider this procedure appropriate.
It seems as though:
- The less people know about genetics, the higher the likelihood that they think behaviors are mere puppets controlled by their puppet master, namely genes — or one single gene.
To make things even more complicated, the way people reason about genes (or single genes) has something to do with their moral standing and political views:
Vote For Genes
In 2013, Elizabeth Suhay and Toby Epstein Jayaratne showed that conservatives are more likely to endorse genetic explanations for perceived race and class differences; liberals are inclined to attribute these differences to structural inequalities or discrimination.
On the other hand:
Liberals more readily used genetic explanations for differences in sexual orientation, whereas conservatives pointed to choice and environmental factors.
Apparently, no ideological position comes with the tendency to attribute individual differences to genetics.
This side-switching procedure suggests that motivated reasoning is at work.
As Jonathan Haidt, psychology professor at NYU, writes in The Righteous Mind:
Intuitions come first, strategic reasoning second.
Genetic determinism, the idea that genes directly control human behaviors, is not limited to the individual…
…it spreads through multiple generations:
The (Genetic) Sins of the Father
Eric Luis Uhlmann and colleagues demonstrated in 2012 that people judge an adopted person as tainted by “the sins of the father” if the biological male parent turned out to be a criminal…
…regardless if the adoptee had ever met the biological parent(!)
The same goes if a man hurt an innocent family:
The offender’s biological, but not the non-biological, grandson was automatically thought of as capable of committing a similar act, pointing to a “moral spillover” effect.
In other words, one is guilty by genetic association
As Menelaus said in The Odyssey:
The blood of your parents is not lost in you.
But how, and to what extent, do genes mediate behaviors?
Worms, Genes & Behaviors
Our genes come from our parents. Genes are the building blocks from which our life springs. So, it is plausible that genes determine our behaviors.
If one looks closer, however, the idea of genetic determinism does not hold water.
Consider the worm Caenorhabditis elegans, or C. elegans for short:
It has roughly 13,000 genes and 1,000 cells — 300 of which are neurons. In the worm, numerous genes are involved in developing its neurons; a myriad of neurons are needed to mediate a particular behavior with frequently overlapping neural circuits.
One gene in C. elegans is involved in the development of, and ultimately in mediating, several behaviors.
What does this mean?
The building process is stochastic, rather than deterministic.
Randomness in the form of environmental factors, both endogenous and exogenous, play a key role in the development of the worm’s neural networks and what behaviors they ultimately influence or not.
The bottom line?
- No single gene is responsible for a particular behavior!
Or in the elegant words of Robert Sapolsky:
Genes are not autonomous agents commanding biological events.
Following this thought:
If there is no single gene that codes for one particular behavior in such a simple organism as C. elegans, it is unlikely that such a gene exists in humans, who are of greater genetic complexity.
This opens up a whole world of questions.
For this reason…
In this article series, we will investigate that complexity by closely examining three general questions:
- How do genes influence behavior?
- To what extent do genes affect human behaviors?
- What are the limits of their power to influence those behaviors?
To begin this process, we need to get a clear understanding of how genes work. All the proteins, enzymes (most enzymes are proteins, but not all of them), and…well..stay tuned!