The problem with equal dollar weights, however, is that to maintain them as stock prices fluctuate requires frequent rebalancing of the portfolio, which incurs transaction costs. Later index funds tended to seek to track the S&P 500 index, which includes stocks not traded on the New York Stock Exchange and, crucially, weights them by market capitalization, so the need for rebalancing is greatly reduced.

Donald MacKenzie, An Engine, Not a Camera, loc. 1482