『A Random Walk Down Wall Street』要約

Burton G. Malkiel’s A Random Walk Down Wall Street, first published in 1973 and revised many times since, argues that stock prices follow a random walk, meaning short-term movements are essentially unpredictable. A Princeton economist and former investment executive, Malkiel builds the book around the efficient market hypothesis and the related idea that most investors cannot consistently beat broad market indexes after costs. He opens with a famous parable distinguishing “castle-in-the-air” investors, who buy on psychology and crowd behavior, from “firm-foundation” investors, who value assets by fundamentals. The book surveys historic manias and bubbles, including tulip speculation, the South Sea Bubble, and the 1929 crash, to show how recurring euphoria undermines valuation discipline. Malkiel introduces several memorable frameworks and tests. His “airport” or “wallflower” analogy asks which stocks look attractive to a blindfolded monkey throwing darts, illustrating indexing. He examines technical analysis and fundamental analysis skeptically, reviews the Capital Asset Pricing Model and beta, and discusses behavioral finance anomalies that challenge strict efficiency. Later chapters address modern portfolio theory, asset allocation, dollar-cost averaging, and the case for low-cost index funds, often citing the performance record of the Vanguard 500 Index Fund. He also evaluates mutual fund managers, market timing, and the costs of active trading. The book concludes with practical, evidence-based advice: diversify globally, hold bonds and stocks in risk-appropriate proportions, rebalance, and keep fees low. Rather than promising a winning formula, Malkiel defends a disciplined, low-cost, buy-and-hold strategy as the most reliable path for ordinary investors.