Army ants

Army ants appear in the book as both an effective collective and a cautionary example of what happens when individual judgment disappears. Their circular mill, in which ants follow one another around a loop until most die, makes visible the danger of unbroken imitation—and sharpens the contrast between a colony’s coordination and the independence that helps human groups reach sound decisions.

The Circular Mill

In the early twentieth century, the naturalist William Beebe encountered army ants moving in a vast circle in the Guyana jungle. The loop measured 1,200 feet in circumference, and each ant took two and a half hours to complete it. The ants continued around and around for two days, until most had died.

Biologists call this pattern a circular mill. It forms when army ants become separated from their colony and lose their way. Each ant follows a simple rule: follow the ant in front. The circle generally breaks only when a few ants happen to stray from the path and others follow them away. Until that chance departure, the behavior of each ant reinforces the same movement in all the others.

Collective Success and Individual Dependence

The circular mill is especially revealing because it contrasts with the colony’s ordinary effectiveness. No single ant commands the colony or directs its work; each individual knows little on its own. Yet, as the book explains through Steven Johnson’s account, the colony finds food, completes its work, and reproduces itself. Coordinated behavior succeeds without a central leader.

The same dependence that supports this collective capacity also creates its vulnerability. In the mill, every ant’s next move depends on what the ants around it do. The rule that ordinarily helps organize activity becomes self-reinforcing: no ant acts independently, and so none breaks the march until some ants leave the loop by chance. Collective coordination, in this instance, does not correct a mistaken course; it perpetuates it.

Significance for Human Judgment

The episode introduces a distinction the book makes between humans and ants. Independence does not mean isolation, but relative freedom from others’ influence: opinions must remain, in some sense, one’s own. The ants’ circular march supplies a stark image of the alternative—following those ahead simply because they are ahead.

That contrast matters to the book’s account of collective judgment. Groups are more likely to make good decisions when members retain independent judgments, since each person can contribute private information, including interpretation, analysis, or intuition. The army ants therefore serve not merely as an illustration of group behavior but as a counterexample: a collective can function remarkably well, yet lose its capacity to correct itself when members cannot depart from one another’s actions.