The "long-term" averages you often see quoted are dangerous to go by. 1. They usually cover about 100 years. If you're starting out your career you've probably

The "long-term" averages you often see quoted are dangerous to go by. 1. They usually cover about 100 years. If you're starting out your career you've probably got about 40-50 years of saving followed by say 20 years of retirement, i.e. your training data is not much more than the period you're trying to predict. 2. That history is primarily over the period of world history that can be summed up as "cheap oil". Personally I wouldn't use it to predict any other time period. 3. If you look at the returns over time, they're pretty much generational. Some generations get fantastic returns from the stock market, while others at best are keeping up with inflation. If you started investing in say 2000 you probably know which category you're in.