Imagine a world without trust, where personal interactions exist at face value and veiled intentions go unnoticed. New research suggests that people with borderline personality disorder (BPD) see the world in this way.

People with BPD typically have trouble controlling urges and keeping friends, and the syndrome hits women disproportionately.

When sufferers of BPD – which is responsible for up to a fifth of all psychiatric inpatients – play a two-person economic game that depends on cooperation, they play as if they are blind to subtle signs of distrust from a player without the condition.

The finding, supported by brain scans showing reduced activity in an area of the brain cued to social norms, could give psychiatrists a better diagnostic tool and a brain area to target with therapy or drugs when treating BDP, says project leader Read Montague, a neuroscientist at Baylor College of Medicine in Houston.


Trust test

The game is simple. Two players, an “investor” and a “trustee”, sit in different rooms where they can’t interact, except through the computer game.

The investor lends up to $20 to the trustee, who triples the investment and then decides how much to give back. So an outlay of $20 would return $60. If the trustee returned $40 – a $20 windfall for each player – an investor might continue her generosity.

If, however, the trustee skimps, repaying only $20 and keeping the rest, future investments may fall in value.

In healthy people and those with BPD, skimping on repayments inevitably happens toward the end of a game. “It’s not a matter of whether cooperation breaks down, but when,” Montague says.

To counter such plummeting investments, healthy trustees learned to coax the cooperation of their partners with excessive returns – say giving back $14 on a $5 investment.

As a result of the good will, the next four investments stayed high, Montague and colleague Brooks King-Casas found. But trustees with BPD did not pick up on this strategy, and as a result their late game returns stayed low.

Diagnostic tool

A functional-MRI brain scanner helped explain the different outcomes. When normal trustees received an excessively low investment – a sign of mistrust – a region of the brain called the insula went into action, compared to its activity during higher, more trusting offers.

The insula responds to internal body signals, such as pain, heart rate and temperature, but new research has hinted at role for the walnut-sized structure in detecting social norms and fairness. When you have deviations in fairness the insula revs into action, Montague says.

Not so among 55 BPD trustees. Their insulae did not discriminate between trusting (high) and untrusting (low) investments.

Because the brains of BPD trustees responded just like normal trustees when divvying up a stingy investment, Montague theorises that people with the disorder don’t pick up on social cues for mistrust. “In other words: ‘I don’t understand your gesture toward me’,” he says.

Peter Bossaerts, a neuroeconomist at Caltech in Pasadena, says that since the games are scored and have a predicted outcome, they may help diagnose mental illnesses.

“This could really revolutionise the way people think about and treat psychiatric illnesses,” he says.

Journal reference: Science (DOI: 10.1126/science.1156902)

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