Jack Treynor

Jack Treynor appears in the book as a finance professor who tested how information affects collective judgment. His jelly-bean experiment begins with a straightforward estimation task and then changes one thing: what participants are told about the jar. The contrast makes his example a warning against assuming that more shared information always improves a crowd’s answer.

The First Estimate

Treynor first asked students in his finance class to estimate the number of jelly beans in a jar. The jar held 850 beans; the class average was 871, within 3 percent of the actual total. Only one student made a better estimate than the group. The result echoed Francis Galton’s ox-weight experiment: individual errors can offset one another, leaving a group average remarkably close to the truth.

When Added Information Distorted Judgment

Treynor then asked the students to estimate the beans again, this time telling them to account for air space at the top of the jar and noting that it was made of plastic rather than glass, so it could hold more beans than they might expect. Both details were true, but the class average became 15 percent inaccurate and performed considerably worse than several individual guesses.

The change was not simply that the students learned something useful about the container. The added information pointed them in a common direction: they were encouraged to imagine that the jar held more beans than they had initially thought. Because many people adjusted their judgments in response to the same framing, their errors no longer canceled out as effectively. In the book’s account, the way information was disclosed mattered as much as its truth.

A Qualification to Crowd Wisdom

Treynor’s experiment does not negate the value of collective judgment; it identifies a condition on it. A crowd can combine pieces of private information only if its members retain sufficiently independent perspectives. Shared commentary can instead synchronize those perspectives and weaken the diversity that makes aggregation useful. In this case, explaining why the plastic jar mattered led the students away from a more accurate estimate.

The episode therefore complicates the book’s broader argument that diverse, independent judgments can produce an accurate collective result. It suggests that public information is necessary in many settings, but that emphatic interpretation can make people react alike rather than contribute distinct assessments. Treynor’s experiment became a compact demonstration of how a group can know less, collectively, after being told more.