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1 APRIL 2005 VOL 308 SCIENCE www.sciencemag.org36
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As any economist will tell you, people don’talways behave rationally when it comes tomoney. For instance, we sometimes trustcomplete strangers with our hard-earneddough. This suggests to many that a ten-dency to trust is hard-wiredinto the human brain.
Until now, little was knownabout the neural circuitryunderlying the capacity totrust. But on page 78, neuro-scientists and economistsfrom Texas and Californiareport an intriguing insight:Activity in a brain regioncalled the caudate nucleusreflects one person’s intentionto trust another with a sum ofmoney. Their results also suggest that trustisn’t purely noble—it may stem from a coldcalculation of expected rewards.
“I think it’s a very important paper. It’sgoing to change the way we think of socialinteractions,” says Paul Zak, who directs theCenter for Neuroeconomic Studies at Clare-mont Graduate University in California.“It’s an exceedingly well done and rigorousstudy,” agrees Paul Glimcher, a neuroscien-tist at New York University.
The research exemplifies the fledglingfield of neuroeconomics, which combines thebrain imaging tools of neuroscience with theexchange games economists have invented toprobe how people behave during financialtransactions. It’s also one of the first studies inwhich the brains of two people were scannedsimultaneously during a social interaction. Twovolunteers played a trust game from insidefunctional magnetic resonance imaging scan-ners, one at the California Institute of Technol-ogy in Pasadena and the other at Baylor Col-lege of Medicine in Houston, Texas.
In each of 10 rounds, one player, the desig-nated “investor,” received $20. The investorthen had the option of sending some, all, ornone of the $20 to the other player, the“trustee.” According to the rules of the game,which were known to both sides, any moneythe trustee received tripled. The trustee thenhad the option of returning a portion of the newsum to the investor. The players’ only knowl-edge of each other came from numbers flashedon a monitor that indicated the amount ofmoney changing hands in each round, as wellas each player’s total for the game.
The extent to which a player trusted anotherwith his or her money depended on the recenthistory of the exchange. If an investor increasedthe contribution to a trustee immediately fol-lowing a round in which the trustee had reducedpayback, the trustee generally rewarded this
benevolent reciprocity with a greater return inthe next round. But if an investor demonstratedmalevolent reciprocity by repaying generositywith stinginess, the trustee usually returned lessthe next time around.
Examining the trustees’ brain scans, theresearchers found that activity in the caudatenucleus was greatest when the investorshowed benevolent reciprocity and mostsubdued when the investor showed malevo-lent reciprocity. Moreover, caudate activityrose and fell with changes in the amount ofmoney trustees returned to their investors on
the subsequent round. The team concludesthat activity in a trustee’s caudate nucleusreflects both the fairness of the investor’sdecisions and the trustee’s intention to repaythose decisions with trust (or not).
The caudate nucleus’s “intention totrust” signal appeared about 14 secondssooner in later rounds of the game, an indi-cator that the trustee is building an opinionof the investor’s trustworthiness, says ReadMontague, who led the Baylor team.
The caudate nucleus is well connected tothe brain’s reward pathways, and previouswork has shown that it revs up when sub-jects expect a reward such as juice or money.Montague and colleagues speculate thattrust, admirable trait that it is, boils down topredicting rewards—in this case, the “socialjuice” of the investor’s reciprocity. Trust hasbeen an element of human social inter-actions for many thousands of years, saysErnst Fehr, a neuroeconomist at the Univer-sity of Zurich in Switzerland, so it makessense that it would tap into ancient neuralsystems like the reward pathways.
–GREGMILLER
Economic Game Shows How the Brain Builds TrustNEUROSC I ENC E
‘Cranky’ Proof Reveals Hidden RegularitiesMathematicians crave patterns, andnowhere do they find richer pickings than inthe theory of numbers. Five years ago, abreakthrough in a long-standing problemconnected with one of the simplest func-tions of number theory yielded an unex-pected bonanza of new patterns. Now, a newproof suggests that that was just the begin-ning. “It’s almost certain that there will be
more where this came from,” says number theorist George Andrews of PennsylvaniaState University, University Park, whosework helped pave the way for the new result.
The proof involves the partition function,which counts the number of ways you can reachany integer by adding other positive integers. Forinstance, the number 4 can be partitioned in fivedifferent ways: 1 + 1 + 1 + 1, 2 + 1 + 1, 2 + 2,
3 + 1, or simply 4 itself. In otherwords, the fourth “partitionnumber” is 5. Similarly, the
fifth partition number is 7.Partitions crop up
throughout numbertheory and haveproved handy forb a l a n c i n genergy budgetsin particlephysics.
In 1910 or so,Indian mathemat-
ical genius SrinivasaRamanujan noticed that
not only the fourth partition number butevery fifth partition number after it is alsodivisible by 5. What’s more, every seventhpartition number (beginning with 7) is divis-ible by 7, and every eleventh partition
MATHEMAT I C S
Tête-à-tête. Brain scans of the investor (left) and trustee in aneconomic exchange game shed light on the neural basis of trust.
3+ 3 + 2 + 1= 9
Sorting it out. “Rank” and “crank” functionsdivide partitions (above, of 9) into classes.
N E W S O F T H E W E E K
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(5718), 36. [doi: 10.1126/science.308.5718.36a]308Science Greg Miller (March 31, 2005) Economic Game Shows How the Brain Builds Trust
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