<prompt> Human: Investment in market indices i.e. SP500 over long periods of time seems to be basic investment advice and really regarded as foolproof. Market will always go up in the long run. What, fundamentally, is the reason behind this saying? Shouldnt growth ultimately be limited or face decreasing marginal growth over time? And if we just say look at historical data -- well the United States has only really been in existence for 250 years I dont know stock market history, but I imagine purchasing and selling with all modern options is a pretty recent development. Maybe 3-4 generations worth of data. How is that enough?Not going against any of this, just curious to understand.  <response>: ELI5 - how can we be so certain of overall market performance in the very long run e.g. Retirement accounts with only really 2-3 generations of historical data?We cant. Investment in market indices i.e. SP500 over long periods of time seems to be basic investment advice and really regarded as foolproof. Market will always go up in the long run. What, fundamentally, is the reason behind this saying?Stocks are derivatives of corporate earnings. Earnings tend to at least increase by the inflation rate over time, otherwise businesses would not be profitable, as they can raise prices for their goods and services. Assuming that not all businesses will fail to be profitable or at least break even, the market will go up by at least the inflation rate. 