I can't tell you how many times people come into our offices and think they make about 10% annually on their investments. After looking a little closer, what they usually see in their returns tells a little different story. One year they'll make 10% but the next year they'll lose 12%. What our brains rememeber though, is the gain and not the loss.
The Wall Street Journal has done a good piece in reference to this myth and some others all should see.