A few decades ago, most buyers and sellers were individual investors, such as wealthy businessmen, usually with long family histories to particular corporations. Over time, markets have become more "institutionalized"; buyers and sellers are largely institutions (e.g., pension funds, insurance companies, mutual funds, index funds, exchange-traded funds, hedge funds, investor groups, banks and various other financial institutions).
Overall commission costs can also be affected by new customer promotions. Brokers may give you a chunk of free trades based on your deposit amount. If your deposit gets you a substantial number of free trades, that can write off otherwise higher per-commission costs. Ally Invest offers small incentives for deposits as low as $500. Fidelity Investments, meanwhile, has a higher barrier for entry — it takes a $50,000 deposit, but then you'll get 300 free trades.
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According to one interpretation of the efficient-market hypothesis (EMH), only changes in fundamental factors, such as the outlook for margins, profits or dividends, ought to affect share prices beyond the short term, where random 'noise' in the system may prevail. The 'hard' efficient-market hypothesis does not explain the cause of events such as the crash in 1987, when the Dow Jones Industrial Average plummeted 22.6 percent—the largest-ever one-day fall in the United States.