By Catherine A. Sanderson, Amherst College
Starting in 2000, with the announcement of the completed draft of the human genome, the total complement of genes found in the nucleus of each human cell, behavioral genetics was suddenly able to examine significantly more complex questions. The field, therefore, shifted from examining whether there are biological bases for behavior to examining how much specific genes influence behavior.
Genes and Psychological Behavior
One of the newest methods used to examine questions in psychology is behavioral genetics. How are genes linked with psychological behaviors? From roughly 1960 to 1990, research in the genetics of behavior was based almost entirely on twin studies, adoption studies, and extended-family studies.
It focussed on questions such as how similar are identical twins when raised in different homes? Are adopted children more similar to their biological parents or adoptive parents? Do particular conditions, such as depression or Alzheimer’s disease, run in families?
These studies did provide some evidence that genes matter, but typically couldn’t tell exactly how or why.
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Interestingly, genetic factors predict how kind we are to other people, whom we vote for in elections, and how likely we are to get divorced. A 2018 study found that some people seem to be genetically predisposed to getting divorced. Adopted children’s rate of divorce was similar to that of their biological parents, even if they were separated from these parents very early in life. There was no correlation between adopted children’s likelihood of divorce and that of their adoptive parents.
Yet, this genetic predisposition is relatively limited in its impact. Genetic factors only predicted 13% of a person’s likelihood of getting a divorce. So, this tells us that genes do play a role, but a relatively small role in predicting something like getting divorced.
Moreover, most complex behaviors are influenced by multiple genes, each gene interacting with other genes, and with environmental factors. Contrary to what was commonly touted back around 2000, it’s not as simple as finding ‘the gene’ that’s responsible for depression, or alcoholism, or your IQ, or whether you will get divorced.
Instead, it’s how genes and the environment interact that predicts behavior. For example, even among people with the same genetic predisposition to obesity,such as siblings, environmental factors may influence whether the gene that triggers obesity is turned on or off.
This finding helps explain why, even with identical twins, one twin may be obese and the other not.
This finding, that environmental factors can cause changes in gene expression, is known as epigenetics. Moreover, these environmental changes in gene expression can be passed on through generations. Here’s a powerful example from women who were pregnant and near the World Trade Centers on the day of the 9/11 terrorist attacks.
Researchers measured these women’s levels of cortisol, a hormone that helps the body respond effectively to stress, shortly after the attack, and about a year later tested symptoms of post-traumatic stress disorder, or PTSD. Women who developed PTSD in the year after the attack had abnormally low levels of initial cortisol compared to those who did not develop this disorder, perhaps exacerbating the longer-term harm of stress for these women.
But here’s how epigenetics makes it even more interesting. The children who gestated inside women who developed PTSD also had abnormally low levels of cortisol, especially if their mothers were in the third trimester of pregnancy at the time of the 9/11 attack.
These babies were basically programmed in the womb to have a lower level of cortisol, as a result of their mother’s traumatic experience, and that likely increased their risk of developing a stress disorder themselves.
The Warrior Gene
These advances in genetics have also led to challenging legal questions, such as whether we should be able to use our genes to excuse bad or criminal behavior. In a 2009 murder trial, a defense attorney provided evidence to a jury that his client was not guilty of premeditated murder because he had the so-called ‘warrior gene’ which has been shown to predispose people to aggressive behavior and difficulty controlling their impulses.
The jury agreed, convicting on charges of manslaughter instead of first- or second-degree murder. As one jury member noted, “Some people without this would react totally different than he would. A bad gene is a bad gene.”
Common Questions about the Connection between Psychology and Genetics
Yes, genetic predisposition is relatively limited in its impact. For example, in a study it was found that genetic factors only predicted 13% of a person’s likelihood of getting a divorce. So, genes do play a role, but a relatively small role in predicting something like getting divorced.
The finding that environmental factors can cause changes in gene expression is known as epigenetics. Interestingly, these environmental changes in gene expression can be passed on through generations.
Most complex behaviors are influenced by multiple genes, each gene interacting with other genes, and with environmental factors. So, it is how genes and the environment interact that predicts behavior. For example, even among people with the same genetic predisposition to obesity, environmental factors may influence whether the gene that triggers obesity is turned on or off.