By Sandra Aamodt and Sam Wang
Sam Wang and Sandra Aamodt
Last Monday, Nicholas Hughes, son of poets Ted Hughes and Sylvia Plath, killed himself. His mother was one of the world’s most famous suicides, and news stories have mentioned the tendency of suicide and depression to run in families. But this tragic inheritance is just part of a more complex story in which our lives are shaped by genes, environment — and unexpected connections between the two.
Much more than depression is partly inherited. Here’s a weirder fact: the genes you get from your parents partly determine your risk of being mugged. So do genes dictate our fate? Of course not — but they do have a say in who we become.
We tend to think of the environment as something that just happens to us, but in fact animals actively seek out surroundings that are compatible with their genetic predispositions. Teenagers in the chess club choose to be exposed to different influences from their hockey-player counterparts. Such differences don’t even have to be voluntary: tall kids may be picked more often for the basketball team and end up better at the game because they have more opportunities to develop their skills.
Certain people are much more likely than others to be exposed to stressful life experiences, including specific traumas like car accidents, industrial injuries or being a crime victim. Some of this variation is traceable to genetics.
Psychiatric geneticists have formalized this idea by studying “heritability,” the amount of the variation within a population that can be explained by genetic differences between individuals. Identical twins are more likely to both experience a variety of life events than fraternal twins, who, like siblings of different ages, share only half their genes. About one-fourth of the variation in life experiences — from strictness of parents to difficulties with friends — can be traced to genetic origins. This finding emerges from dozens of studies.
People whose identical twins are alcoholic — whether or not they themselves have any substance abuse problems — are more likely to have been robbed or gotten in trouble with the law than people whose fraternal twin is alcoholic. It’s easy to imagine that someone who is impulsive and prone to addiction would be more likely to get into bar fights than someone who has neither of those characteristics.
In other words, people with similar personalities seek out similar experience and may take similar risks. For example, if you are the type of person who seeks out excitement, you might be more inclined to walk through shady neighborhoods — placing you at greater risk of being mugged.
What connects our genetic inheritance to environmental experiences? Most likely it is personality, which is known to depend on genes. In one study, three common measures of personality — extraversion, neuroticism and openness to experience — were enough to explain the entire heritability of some life events. In general, neurotic people are more likely to experience negative life events, while extraverted people are more likely to experience positive and controllable life events.
So some of the effects that we call “genetic” (or “nature”) are the indirect result of people being drawn to particular environments because of their personality. Or to put it another way, some “environmental” (or “nurture”) effects are actually attributable to genetic tendencies.
This seeming paradox underscores the point that the “genes versus environment” debate is asking the wrong question. It is said that parents of one child believe that upbringing determines personality, but parents with two children believe in genetic tendencies. The evidence points to something more complex: genetic predispositions interact with circumstances to produce unique individuals.
Now, back to Nicholas Hughes. Major depression arises from a vicious cycle between genes and environment. Let’s start with genetics: a particular gene influences the sensitivity of individuals to bad experiences. One famous paper demonstrated a complex interaction between the serotonin transporter gene and negative events. (The gene encodes a protein that removes the neurotransmitter serotonin from the synapse after a neuron releases it. The action of this protein is inhibited by antidepressants like Prozac.) People with two copies of the high-risk variant of the gene are likely to develop depression in response to multiple stressful experiences like divorce or assault, but they are fine if their environment remains benign.
In contrast, people with two copies of the low-risk form of the gene are resilient against depression, even when they experience environmental stressors. People with one copy of each variant fall somewhere in between, as you might expect.
Genes that predispose people to depression, though, also influence their risk of experiencing negative environmental events. In one study, women whose identical twin suffered from depression were significantly more likely to have been assaulted, lost a job, divorced, or had a serious illness or major financial problems than people whose fraternal twin was depressed. (It’s not known which genes are responsible for this effect.) These bad events did not occur because the women were depressed, as the correlations persisted even when women who were currently depressed were excluded from the study. Thus, genes can act on the same disorder by making people more sensitive to stressful environmental events and by making these events more likely to occur.
The interaction between genetic tendencies and life experiences may explain another puzzling finding: the heritability of many psychological traits — from intelligence to anxiety — increases as people mature. This result seems odd at first glance, since genes are most important in brain development in babies and children. But children also have less control over their environment than adults. As people get older, they become more able to determine their own circumstances, and they may be able to choose environments that reinforce their natural personality tendencies. Apparently those of us who suspect we are turning into our parents as we get older may have a valid point.
After all this, you may wonder if your genes are ultimately to blame for your fortunes, good or ill. That’s hardly the case: only one-fourth of the variation in life events is heritable, which means that three-fourths is not. So you have plenty of opportunity to influence your circumstances. Whether that’s better than turning into your parents, we’ll leave to your judgment.
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NOTES:
We wrote about the neuroscience of personality, risk-taking, and depression in “Welcome to Your Brain.” In addition, see the following references:
For detailed references on people who are prone to experience traumatic events and the heritability of life experiences, see Kendler, K.S. & Baker, J.H. “Genetic influences on measures of the environment: a systematic review.” Psychological Medicine 37:615-626 (2007).
Plomin. R., Lichtenstein, P., Pedersen, N. McClearn, G.E. & Nesselroade, J.R. “Genetic influences on life events during the last half of the life span.” Psychology and Aging 5:25-30 (1990).
Kendler, K.S. & Karkowski-Shuman, L. “Stressful life events and genetic liability to major depression: genetic control of exposure to the environment?” Psychological Medicine 27:539-547 (1997).
Saudino, K.J., Pedersen, N.L., Lichtenstein, P., McClearn, G.E. & Plomin, R. “Can personality explain genetic influences on life events?” Journal of Personality and Social Psychology 72:196-206 (1997).
Magnus, K., Diener, E., Fujita, F. & Pavot, W. “Extraversion and neuroticism as predictors of objective life events: a longitudinal analysis.” Journal of Personality and Social Psychology 65:1046-1053 (1995).
Caspi, A. et al. “Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene.” Science 301:386-389 (2003).
Bergen, S.E., Gardner, C.O. & Kendler, K.S. “Age-related changes in heritability of behavioral phenotypes over adolescence and young adulthood: a meta-analysis.” Twin Research and Human Genetics, 10: 423-433 (2007).
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