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Why is day trading considered riskier than long-term trading?
All forms of liquid investing necessarily have the same expected value. If any one form were more profitable, money would flood in, equalizing it. Day trading is unusual in two key ways. First, although the expected value is the same, the risk profile is very different. For example, would you wager a dollar on the flip of a coin?  You might.  Why not, after all? Would you wager a million dollars?  Probably not. The risk is too great. Similarly, day trading can easily lose you all of your investment, which is why you should be careful doing it. (In his memoirs Liar's Poker, Michael Lewis tells an anecdote about a rich bond trader who proposes a million-dollar, even-money bet with his rival, an amount both could just barely afford to lose.  The rival, not wanting to play but not wanting to lose face by declining, accepted.. with the proviso that the stakes be raised to 10 million dollars! The trader backed down.) Also, the efficient market only guarantees the price will be efficient.  It says nothing about transaction costs.  A busy day-trader can easily incur thousands in commission and other fees.