You’ve likely seen the video before:
a stream of kids, confronted with a single, alluring marshmallow. If
they can resist eating it for 15 minutes, they’ll get two. Some do.
Others cave almost immediately.
This “Marshmallow Test,”
first conducted in the 1960s, perfectly illustrates the ongoing war
between impulsivity and self-control. The kids have to tamp down their
immediate desires and focus on long-term goals—an ability that
correlates with their later health, wealth, and academic success, and
that is supposedly controlled by the front part of the brain. But a new study by Alexander Soutschek
at the University of Zurich suggests that self-control is also
influenced by another brain region—and one that casts this ability in a
different light.
Press your right index finger to the top of your
right ear, where it meets your head. Now move up an inch and back an
inch. You’re now pointing at your right temporoparietal junction (rTPJ). This area has long been linked to empathy and selflessness. But Soutschek, by using magnetic fields to briefly shut down the rTPJ, has shown that it’s also involved in self-control.
Which makes perfect sense. Empathy depends on
your ability to overcome your own perspective, appreciate someone
else’s, and step into their shoes. Self-control is essentially the same
skill, except that those other shoes belong to your future self—a
removed and hypothetical entity who might as well be a different person.
So think of self-control as a kind of temporal selflessness. It’s
Present You taking a hit to help out Future You.
“For a long time, people have speculated that we
use the same mechanisms to reason about other people as about our
hypothetical selves,” says Rebecca Saxe from MIT. “So this new study
fits really well.”
Saxe should know. She was one of the first
scientists to link the rTPJ to theory of mind—the ability to understand
the mental states of other people. In 2005, she and Nancy Kanwisher
scanned people’s brains while they listened to stories in which
protagonists made poor choices based on false beliefs. This experiment
showed that the TPJ is active specifically when people are “reasoning
about the contents of another person’s minds”—the essence of theory of
mind. This region, the duo wrote, helps people to think about thinking
people.
At the same time, many other neuroscientists
were doing similar experiments and getting the same answers. The
consensus was striking, Saxe later wrote.
“Because there was almost no pre-existing neuroscience of theory of
mind, researchers came to the topic with unusually few preconceptions
about where to look in the brain. In those circumstances, neuroimaging
is notoriously fickle, producing many false positives and false
negatives. Yet every group that sought to identify brain regions
implicated in ToM got essentially the same answer; and in study after
study, we still do.”
Many other studies have since expanded on those
early results. If the rTPJ is bigger, people are more likely to behave altruistically. If the neurons within it are better-connected (and well-linked to other parts of the brain), people show less bias towards their own in-groups. If the area is stimulated by electric currents, people become better at taking someone else’s perspective.
And if the region is disrupted, it changes our ability to reason about morality.
Consider a woman who poisons her friend’s coffee—if she does so
deliberately, we’d judge her more harshly than if she acted
accidentally. Intent matters, and we need the rTPJ to judge intent. When
Liane Young, one of Saxe’s former students, disrupted the rTPJ
using magnetic fields, she found that people were more lenient towards
the deliberate poisoner, as long as her friend survived. With their
ability to gauge intent disrupted, they started looking to outcomes
instead.
Not everything fits with the idea of the rTPJ as
a nexus for theory of mind. For example, many studies suggest that it
affects our ability to shift our attention
from one part of space to another, like a technician moving a spotlight
around. “Even in my own small lab, people disagree about the function
of the rTPJ,” says Young, now a professor at Boston College.
If you look at the debate and relax your eyes, you can probably merge the two viewpoints into one. Maybe the rTPJ is a region that redirects our attention from one thing to another—whether
between objects in the world around us, or between our minds and other
people’s. Alternatively, it’s likely that what we call the rTPJ is not
actually a singular bit of the brain. “There’s a lot of work suggesting
that there are different sub-regions—one of which does spatial
reorienting, and the other does perspective-taking,” says Young.
That’s where Soutschek’s study comes in.
He specifically focused on the back half of the rTPJ—the one that’s
been more heavily linked to empathy—and disrupted it in 43 volunteers.
When that happened, the recruits became more likely to pocket a pile of
cash for themselves rather than splitting it with a partner, and
especially when the partner was a stranger. But they were also more likely to pick a small immediate lump of cash over a larger future one, especially when the delays were long.
A second experiment explained why. This time,
the volunteers saw a picture of a man standing in a room with red discs
on the wall. The volunteers could see all the discs, but they had to say
how many the man in the room could see. They had to shift their
perspective to his, and they became worse at that when their rTPJ was
disrupted. What’s more, Soutschek showed that the extent of their
bias—their inability to leave their own heads—predicted both how
impulsive and how selfish they were in the earlier experiment.
This tells us that impulsivity and selfishness
are just two halves of the same coin, as are their opposites restraint
and empathy. Perhaps this is why people who show dark traits
like psychopathy and sadism score low on empathy but high on
impulsivity. Perhaps it’s why impulsivity correlates with slips among
recovering addicts, while empathy correlates with longer bouts of abstinence.
These qualities represent our successes and failures at escaping our
own egocentric bubbles, and understanding the lives of others—even when
those others wear our own older faces.
Ed Yong is a staff writer at The Atlantic, where he covers science.
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