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.