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Contributing factors to historical increase in trading volume
It's not primarily more people investing. In the 1980s stock exchanges went from open outcry trading floors where all trades involved actually exchanging pieces of paper to electronic trading. Once that happened, it wasn't long before most trades were executed by computer programs rather than human beings, turning stocks over rapidly for very short-term profits rather than long-term investment, greatly increasing  the number of trades (and also increasing liquidity for the actual investors; it's by no means all bad).