flocks of birds, traffic jams, fads, drinking games, forest fires and
residential segregation have in common? The answer could come from a
new computational research method called agent-based modeling.
Macy, a sociologist at Cornell University, Ithaca, N.Y., is using this
powerful new tool to look for elementary principles of self-organization
that might shed new light on long-standing puzzles about how humans
interact. A professor and chair of Cornell's Department of Sociology,
Macy will speak Feb. 14 at the annual meeting of the American Association
for the Advancement of Science in Denver in a symposium, "Artificial
Agent Societies: A Computational Future for the Social Sciences."
sociologist begins his lecture with a flock of computer-generated birds
wheeling synchronously through aerobatic maneuvers. He credits Craig
Reynolds, a pioneer of agent modeling and three-dimensional computer
animation, for the 1987 discovery that the complex choreography of a
flock requires that each bird (or "boid," as Reynolds called them) follow
just three simple rules: head toward the center of your neighbors, match
their speed and trajectory and avoid collisions. "Reynolds didn't model
the flock as a unitary collective nor did he model isolated birds; he
modeled their interactions at the relational level," Macy says. "That's
sociologists have tried to understand social life as a structured system
of institutions and norms that shape individual behavior from the top
down, Macy notes. In contrast, agent modelers suspect that much of social
life emerges from the bottom up, more like improvisational jazz than
a symphony. For example, many sociologists have attributed residential
segregation to the deliberate policies of banks, realtors and public
officials responding to popular prejudice. Yet 30 years ago, game theorist
Thomas Schelling used one of the first agent-based models to show how
extreme segregation tends to emerge even in populations that prefer
ethnic diversity, and in the absence of any institutional pressures.
model was written long before the invention of the personal computer,
using a large checkerboard, with red and blue poker chips to represent
a neighborhood's residents. Even 10 years ago agent-based models with
large populations might have required hundreds of costly hours on supercomputers;
but today's fast and inexpensive desktop machines make this method readily
available to social scientists. These models are now being used to show
how "simple but predictable local interactions among many individuals
can generate familiar but enigmatic social patterns, such as stock market
crashes, revolutions, fads and feeding frenzies,"says Macy.
with his Cornell colleague David Strang, Macy has used agent-based models
to study lemminglike fads among the corporate managers pilloried by
Scott Adams in his "Dilbert" comic strip. Contrary to Adams, top managers
are highly intelligent and are paid huge salaries to get it right, Macy
observes. The Cornell researchers' work, which won the theory prize
from the Academy of Management, shows how fads that appear to reflect
mindless conformity can be generated by the very opposite -- a single-minded
preoccupation with performance and success.
was awarded a grant from the National Science Foundation to tackle another
familiar puzzle -- peer enforcement of norms that even the enforcers
privately question. In collaboration with two Cornell graduate students,
Robb Willer and Damon Centola, Macy is using an agent-based model to
see how the diffusion and stability of unpopular norms might depend
on the size and geometry of peer networks. The father of teenagers,
Macy ponders the curious appeal of self-destructive behaviors -- smoking,
drinking, drug use, reckless driving, body-piercing and the like. For
example, studies of college drinking find that students feel peer pressure
to participate in drinking rituals that celebrate intoxication as a
symbol of group identity.
turns out that students' private beliefs deviate sharply from their
perception of the social norm," he notes. "Contrary to campus legend,
most students are actually uncomfortable about excessive drinking, at
least when they are sober. They do not think drinking games are cool,
but they think (incorrectly) that others believe this, and when they
join in to secure social approval, their apparent enthusiasm reinforces
the illusion that motivates the behavior in others."
) tried to generate this dynamic on a computer, the agents always escaped
the trap. But then the researchers remembered a lesson taught by Reynolds'
boids. "The boids only know about the behavior of their immediate neighbors,
and that turns out to be the key to the puzzle," Macy says. "Agents
get trapped into enforcing a norm that most of them dislike when their
normative expectations are mainly influenced by a small circle of friends."