What is What Works on Wall Street about?

What Works on Wall Street is James O'Shaughnessy's systematic examination of stock market returns across multiple decades, analyzing which investment strategies actually produce superior long-term results and which are myths. Originally published in 1996, the book has been updated through four editions, most recently in 2011, each adding more years of data and refining the conclusions. It is one of the most data-intensive treatments of practical investing available to a general audience.

The methodology is empirical and Compustat-based: O'Shaughnessy took decades of stock market data and back-tested a large number of investment strategies — growth, value, momentum, income, combination approaches — to determine which produced risk-adjusted returns above the market average over time. The results are often counterintuitive. Many strategies that investors believe work — buying stocks with recent strong performance, chasing earnings momentum, buying growth at any price — underperform the market over full cycles. Others that are theoretically straightforward but behaviorally difficult — buying stocks with low price-to-sales ratios, combining value and momentum factors — produce consistent long-term outperformance.

O'Shaughnessy's key finding is that quantitative strategies outperform discretionary stock picking not because human judgment is worthless but because human judgment is subject to behavioral biases — overconfidence, loss aversion, recency bias — that cause consistent deviation from optimal decision rules. A mechanical system that removes human judgment from the execution of a proven strategy produces better outcomes than the same strategy executed with discretion, because discretion introduces noise.

The book is primarily for sophisticated investors and has a technical register that may be challenging for readers without background in financial statements or statistics. It is also a product of a specific era: the strategies it identifies were back-tested on historical data, and there is a live debate about whether published factors persist after discovery because markets adapt. These are genuine limitations. What Works on Wall Street remains, however, the most thorough empirical treatment of long-term equity investing strategies available to an individual investor.