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Contributing factors to historical increase in trading volume
Prior to 1975, commissions for trading stock on the NYSE were fixed at 1% of the amount of the trade.  In 1975, the SEC made fixed commission rates illegal, giving rise to discount brokers offering much reduced commission rates.  Simultaneously, Electronic Communications Networks (ECNs) gained market share as alternative venues for executing trades.  The increased competition led to further declines in commissions.  Finally, as technology was widely adopted on Wall Street and human beings were largely taken out of the order execution process, commissions fell further.  This had the effect of both drawing in new participants and increasing the rate of transactions of the existing participants (see Day Trading, which was largely unheard of prior to the technology revolution of the 1990s).  Most recently, the exchanges themselves have shifted their business model to depend on high frequency traders, and the proportion of trades accounted for by HFT firms ballooned from under 10% in the early 2000s to over 50% today.