March 28, 2009

Mugged by Our Genes?

Baby Climbing Strand of DNA
© Photographer: Aliencat | Agency:
Mugged By Our Genes?
March 24, 2009

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

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

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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