Anatomy of Give and Take
HOUSTON — The two women had money in mind.

Phuong Tang, 25, wriggled into a $2.5-million brain scanner at Baylor College of Medicine. Across the hall, a technician loaded Tang's trading partner for the day — Kavita Belur, 26 — into the bore of a similar machine, like a fresh artillery shell.

The two strangers were speculators in a marketplace of the mind, locked in a mutual struggle for financial gain. Belur played an investor, Tang the trustee of an investment fund.

As the pair wavered between cooperation and betrayal, scientists recorded how their brains changed. The researchers hoped to discover the secret of trust — the human variable missing from the mathematics of modern economics.

The terms of the experiment were simple: At the beginning of each round, Belur could put up to $20 in play. Any investment automatically tripled. Tang then decided how much to return and how much to keep.

Belur's safest strategy was to hoard all of her money. Tang's most logical move was to cheat her partner at every opportunity.

There was a riskier but potentially more profitable way.

They could trust each other.

The experiment was part of a new frontier in the exploration of the brain — a field called neuro- economics that seeks to understand the biology underlying economic behavior.

In universities and research centers across the country, scientists are probing the brain with coin flips, $5 bills and gift certificates from Bit by bit, they are assembling a mosaic of the financial brain, identifying how competing neural circuits shape decisions.

"We have started looking for pieces of economic theory in the brain," said New York University neuroscientist Paul Glimcher.

Researchers believe they can discover how neural networks affect the ways people buy and sell, splurge and save. They hope one day to understand how decisions percolating through the brains of billions of people, often acting at cross-purposes, interact to chart the course of financial markets and national economies.

Inside the scanner, Belur made up her mind.

She decided to gamble her entire nest egg on her trustee's goodwill.

She pushed the button, putting her money in play.

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With the ritual clang of the opening bell one day in February, the five trading floors of the New York Stock Exchange abruptly surged in a whirlpool of profit and loss.

Hundreds of brokers waved cellphones, fingered small computer keypads and placed their clients' orders. Fortunes winked into existence and just as quickly vanished.

In all, about 1.6 billion shares — worth about $46 billion — changed hands during the day in a ripple of deals coursing through the global equities market. The daily behavior of buyers and sellers is so complex that even experts in chaos theory have been unable to discern a predictable pattern.