The late-1990s tech-stock bubble was a broad episode of speculative market enthusiasm in which prices lost their connection to underlying value: investors bid shares higher because they expected them to keep rising, until the rush reversed. In the book’s account, the episode shows that markets do not automatically turn many judgments into wisdom. When investors’ expectations converge and dissent is difficult to express, collective decision-making can send capital toward the wrong companies and magnify the eventual correction.

From sector manias to a broad frenzy

The book places the tech-stock bubble in a history of market enthusiasms that had often been confined to particular industries. Earlier examples included uranium shares, buoyed by the promise of the Atomic Age; conglomerates in the mid-1960s; recreational-vehicle stocks; and later waves involving personal computers, biotechnology, and real estate. The distinction is one of scale: a sector bubble can leave most investors untouched, while a market-wide frenzy draws a much broader public into the same expectations.

The late-1990s tech-stock episode belongs to this more consequential category. The book describes it alongside the South Sea Bubble of the 1720s and Japan’s 1980s real-estate boom, when a single piece of land in Tokyo was said to be worth more than all of California. In each case, the comparison concerns a moment when prices and value part company. Investors keep buying because they expect prices to rise, and the prospect of continued increases becomes a substitute for evaluating what the assets are worth.

How the bubble changes market judgment

In the book’s account, bubbles reverse the conditions associated with accurate collective judgment. A wise crowd depends on diversity of opinion, independence, decentralized knowledge, and a means of aggregating private information. During a bubble, independence, diversity, and private judgment give way: participants are swept into a shared expectation instead of contributing distinct assessments. The market may still combine many transactions, but its apparent consensus no longer guarantees that it is drawing on a wide range of information.

This failure matters beyond investors’ portfolios because stock prices help direct capital. A company with a high share price can raise more money by selling stock or issuing bonds. When prices reflect sound collective judgment, companies that attract capital can use it productively, benefiting both the firms and the wider economy. When a bubble pushes prices far above value, that same mechanism directs too much money toward favored companies and away from others.

Short selling and the limits of correction

The book connects the danger of distorted prices to the shortage of short sellers. It does not present short sellers as uniquely brilliant or invariably correct; rather, their ability to take positions against a stock can add viewpoints that a market otherwise lacks. A market in which people can bet on only one side of a proposition might still reach a correct answer, but its participants would be more likely to share biases and make the same mistakes. If those one-sided bets were wrong, the resulting distortion could be greater.

The same concern applies to stocks that are difficult to short. The book says Internet stocks were almost impossible to short, a constraint that may help explain why their prices rose so far. The point is not that short selling guarantees accurate prices, but that suppressing contrary views can make a wrong price more extreme. In this account, the absence of short selling—not short selling itself—is the greater danger to a market’s capacity to correct its errors.

The bubble as a warning about collective decisions

The tech-stock bubble is presented as evidence that collective decisions can fail when the mechanisms that preserve independent and varied judgment break down. Such failures are unusual compared with ordinary market behavior, but their economic consequences can be large because prices influence where investment goes. The bubble therefore complicates any simple celebration of crowds: aggregation can produce a powerful collective estimate, yet a crowd that follows a common expectation may lose the very diversity and independence that make its judgment valuable.