"Interestingly, participants in this experiment were unable to articulate why they behaved the way they did," Zak said. "But nonetheless their brains guided them to behave in 'socially desirable' ways — that is, to be trustworthy."
Inside the Baylor scanner, Belur invested another $20.
She signaled her decision, then awaited Tang's next move.
Was trust its own reward?
When a decision forms, the brain moves faster than self-awareness.
The brain unconsciously prepares to act a measurable length of time — up to 500 milliseconds — before a person consciously decides to act.
In other words, the brain is always one step ahead of itself, calculating the potential costs and benefits of each choice at a cellular level.
"Most of the brain is dominated by automatic processes, rather than deliberative [thinking]. A lot of what happens in the brain is emotional, not cognitive," said George Loewenstein, a behavioral economist at Carnegie Mellon University.
Some brain cells are especially sensitive to the potential rewards of decisions, research at Baylor and Emory University suggests.
Brain cells that release a chemical called dopamine, which serves as a reward to reinforce behavior, actually anticipate snap decisions to help balance costs and payoffs. The cells secrete a burst of good feeling beforehand to underline the desirability of one course of action versus another.
These neurons respond selectively. Some react only to the possibility of something beneficial and others only to the reward itself, researchers at the University of Fribourg in Switzerland discovered.
Every brain is of two minds about the future.
Two competing neural systems interact during choices that hinge on a conflict between short-term and long-term benefits, Harvard University researchers reported.
"Our emotional brain has a hard time imagining the future, even though our logical brain clearly sees the future consequences of our current actions," said Harvard economist David Laibson. "Our emotional brain wants to max out the credit card, even though our logical brain knows we should save for retirement."
Moral dilemmas can engage the same sense of fair dealing and mutual obligation as money matters. Researchers at Princeton University determined that synapses active during complex moral choices tapped into areas associated with rational thinking — and also into regions aroused by strong emotion.
"Some of that emotional architecture affects decisions we make involving money," Zak said.
Critics have often argued that volunteers playing experimental games in brain scanners are no measure of real market behavior. So researchers led by Lo at MIT studied working traders during their normal business day.
To measure brain activity indirectly, he wired 10 currency speculators at a Boston brokerage to sensors monitoring heart rate, breathing, blood pressure, body temperature and skin conductivity. By the end of the day, the traders had made 1,200 split-second trades, averaging $3 million to $5 million apiece.
His team plotted the biological indicators of stress, exuberance and tension against real-time profit and loss. He repeated the experiment at the Boston Stock Exchange.
Market trades, the sensors showed, were the stuff of sweaty palms, heavy breathing and pounding pulses. Snap judgments, honed by intuition, outweighed high-minded economic calculations.
These were "gut" decisions.
Contrary to traditional economics — which considers only rational deliberation — such measures of market panic and exultation begin to document how involuntary emotions affect the rise and fall of stocks.
Already, preliminary findings about the balance sheet of the brain have scholars rethinking the meaning of money itself.
The same reward circuitry activated by cocaine, sports cars, attractive faces and jokes is activated by money. Until now, economists have assumed that money was prized not for itself but only for what it could buy.
Moreover, the prospect of winning money activates specific brain regions in a way that the threat of losing it does not, researchers at Stanford University recently demonstrated.
Scientists are not sure how the electrical snap of synapses adds up to a financial decision, or how these insights might be assembled into a working theory of economic behavior.
"Sooner or later, you have to engage the issue of free will," said Glimcher, the New York University scientist. "When we finally understand the human brain, all human behavior will be predictable."
For nine rounds, the two women played in perfect trust.
Belur as the investor always put up the maximum possible. Tang, the trustee, in turn always equally divided the spoils.
Now, in the last round, the odds of betrayal reached their peak.
They had both reached the moment when economic theory suggested that the optimal move was for the trustee to seize all the profits because there would no longer be any way for the investor to retaliate.
Tang could cheat her partner without fear of reprisal.
The most rational move for Belur, therefore, was to refuse to risk any money in this last round, to end the game richer than her trading partner.
The women balanced on the cusp of betrayal.
Belur gambled again and put her entire stake in play.
The next 10 seconds of indecision seemed an eternity.
For one last time, Tang evenly split the proceeds of the investment.
"Perfect cooperation every round," said Baylor neuroscientist Damon Tomlin, who was monitoring the experiment from the control room.
The two women eased themselves out of the scanners, stiff and a bit dazed.
Unknowingly, they had defied the rules of game theory. They should have betrayed each other at the earliest opportunity. Had trust hormones triumphed over the theorem of self-interest?
By playing together in such harmony, each had earned 300 points, meaning each would be paid $30.
Tomlin counted out the one-dollar bills from a small lock box.
Tang eyed the growing stack of bills.
"If I had known it was the last round," she told Belur, "I would not have given you anything."
Tang could not explain why she lost track of her strategy, and it puzzled her.
There was no way she could know whether — in the instant of decision — the internal compass of her brain had altered her choice.
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