It's important not to expect too much
from the securities market — such as double digit returns
on your investment every year or a portfolio that never loses
value. However, it's equally important not to expect too
little.
If you invest very conservatively —
or don't invest at all — because you fear losing some
of your principal,
you run the risk of not meeting your goals and even running out
of money during retirement.
Investing almost always entails a certain
amount of risk. But that doesn't mean taking on risk blindly
— it means anticipating what the risks of a certain investment
are and having a strategy in place to manage, or offset, them.
Understanding the ways different types of investments have performed
historically can help you gauge what you can reasonably expect
as an investor, and manage your portfolio accordingly.
Jeremy Siegel, The Wharton School 
