<prompt> Human: I remember him telling me he lost over 50,000 dollars the day the planes struck those towers when i was younger ...why does a national catastrophe like this effect 401ks so badly?  <response>: Value of the account goes up and down with stock prices, but he doesnt LOSE anything unless he sells at a down price. People who held their mutual funds after 911 made back all the loss and more. Those who panicked and sold might have indeed lost money and sat on the sidelines far too long, so missed out on future gains as well. 