Brain Imaging Studies Reveal Biological Basis for Human Cooperation
scans have revealed a "biologically embedded" basis for altruistic behavior,
with several characteristic regions of the brain being activated when
players of a game called "Prisoner's Dilemma" decide to trust each other
and cooperate, rather than betray each other for immediate gain, say
researchers from Emory University. They report on their study in the
July 18 issue of the journal Neuron, published by Cell Press.
For many years, evolutionary
biologists, behaviorists, economists and political scientists have attempted
to understand why cooperation exists between human beings, even though
that cooperation may not result in a direct or immediate reward. This
unselfish behavior called "altruism" is almost uniquely a human trait.
Up until now, almost all
brain imaging experiments that have studied the social brain have done
so by exposing subjects to static 2-D images inside the scanner. "This
study represents an attempt to learn about the social brain by scanning
people as they are engaged in a true social interaction," said James
K. Rilling, Ph.D., principal investigator in the Emory study, who is
currently serving a postdoctoral fellowship at Princeton University.
In the Fall of 2003, Dr. Rilling will return to Emory as a faculty member
with a joint appointment in the Center for Behavioral Neuroscience (CBN)
at Emory University School of Medicine and the Emory University Department
In two separate experiments,
the researchers used fMRI to scan the brains of 36 women while they
played the "Prisoner's Dilemma Game," a decades-old model for cooperation
based on reciprocal altruism. Two players independently chose to either
cooperate with each other or not (defect), and each was awarded a sum
of money that depended upon the interaction of both players' choices
in that round.
In the first experiment,
19 subjects were scanned in four game sessions designed to observe neural
function during cooperation and non-cooperation during both human interactions
(social) and interactions with a computer (non-social). The results
of the first experiment revealed different patterns of neural activation
depending on whether the playing partner was identified as a human or
a computer. In the second experiment, 17 subjects were scanned during
three game sessions, focusing specifically on human interaction.
Mutual cooperation was the
most common outcome in games played with presumed human partners in
both experiments, even though a player was maximally rewarded for defecting
when the other player cooperated. During the mutually cooperative social
interactions, activation was noted in those areas of the brain that
are linked to reward processing: the nucleus accumbens, the caudate
nucleus, ventromedial frontal/orbitofrontal cortex and rostral anterior
"Our study shows, for the
first time, that social cooperation is intrinsically rewarding to the
human brain, even in the face of pressures to the contrary, " said Gregory
S. Berns, M.D., Ph.D., co-investigator and associate professor of psychiatry
in the Emory University School of Medicine Department of Psychiatry
and Behavioral Sciences and member of the CBN. "It suggests that the
altruistic drive to cooperate is biologically embedded-- either genetically
programmed or acquired through socialization during childhood and adolescence."
"Reciprocal altruism activates
a reward circuit, and this activation may often be sufficiently reinforcing
to override subsequent temptations to accept but not reciprocate altruism.
This may be what motivates us to persist with cooperative social interactions
and reap the benefits of sustained mutual cooperation," said Dr. Rilling.
"The combination of game
behavior and functional brain imaging also provides a unique paradigm
to explore the neural basis of social behavioral disorders such as autism,
drug addiction and sociopathy, that are characterized by deficits in
social reciprocity, impulse regulation, or social reward processing,"
adds Clint Kilts, Ph.D., co-investigator and associate professor of
psychiatry at Emory. "It defines the most complex form of the human
genesis of a social bond. It may help us define why wars are fought
and loves are lost."
The study was sponsored by
the Markey Center for Neurological Sciences Fellowship, National Institute
on Drug Abuse (NIDA), National Institutes of Mental Health (NIMH) and
National Alliance for Research on Schizophrenia and Depression (NARSAD).
Other Emory researchers involved in the study were David A. Gutman,
Thorsten R. Zeh, and Giuseppe Pagnoni, Ph.D.