Bayesian Investor Blog

This book does an excellent job of reporting important evidence showing that group decisions can be wiser than those of any one individual. He makes some good attempts to describe what conditions cause groups to be wiser than individuals, but when he goes beyond reporting academic research, the quality of the book declines. He exaggerates enough to give critics excuses to reject the valuable parts of the book.
He lists four conditions that he claims determine whether groups are wiser than their individual members. I’m uncertain whether the conditions he lists are sufficient. I would have added something explicit about the need to minimize biases. It’s unclear whether that condition follows from his independence condition, partly because he’s a bit vague about whether he uses independence in the strong sense that statisticians do or whether he’s speaking more colloquially.
Sometimes he ignores those conditions and makes unconvincing blanket statements that larger groups will produce wiser decisions.
He makes exaggerated claims for the idea that crowds are wise due to information possessed by lots of average people rather than the influence of a few wise people. For instance, he disputes a Forsythe et al. paper which argues that a small number of “marginal traders” in a market to predict the 1988 presidential vote were responsible for the price accuracy. Surowiecki’s rejection of this argument depends on a claim that “two investors with the same amount of capital have the same influence on market prices”. But that looks false. For example, if the nonmarginal traders make all their trades on the first day and then blindly hold for a year, and the marginal traders trade with each other over that year in response to new information, prices on most days will be determined by the marginal traders.
It’s not designed to be an investment advice book, but if judged solely as a book on investment, I’d say it ranks in the top ten. It does a very good job of explaining both what’s right and what’s wrong with the random walk theory of the stock market.
He does a good job of ridiculing the “cult of the CEO” whereby most of a company’s value is attributed to its CEO (at least in the U.S.). I was surprised by his report that 95% of investors said they would buy stocks based on their opinion of the CEO. They certainly didn’t get that attitude from successful investors (who seem to do that only in rare cases where they are able to talk at length with the CEO). But his claim that “Corporate profit margins did not increase over the course of the 1990s, even as executive compensation was soaring” looks false, as well as being of questionable relevance to his points about executives being overvalued. And I wish he had also applied his argument to beliefs of the form “if we could just elect a good person to lead the nation”.
Chapter 6 does a good job of combining the best ideas from Wright’s book Nonzero and Fukuyama’s Trust (oddly, he doesn’t cite Trust).
He exaggerates reports that the stock market responded accurately to the Challenger explosion before any public reports indicated the cause. He claims “within a half hour of the shuttle blowing up, the stock market knew what company was responsible.” I don’t know where he gets the “half hour” time period. The paper he cites as the source says the market “pinpointed” Thiokol as the culprit “within an hour”, but it exaggerates a bit. If the percent decline in stock price is the best criterion, then the market provided strong evidence within an hour. If the dollar value of the loss of market capitalization is the best criterion, then the evidence was weak after one hour but strong within four hours.
He also claims “Savvy insiders alone did not cause that first-day drop in Thiokol’s price.”, but shows no sign that he could know whether this is true. He seems to base on the absence of reported selling by executives whom the law requires to report such selling, but he appears to overestimate how reliably that law is obeyed, and to ignore a large number of non-executive insiders (e.g. engineers). He does pass on a nice quote which better illustrates our understanding of these issues: “While markets appear to work in practice, we are not sure how they work in theory.”