According to the latest data from the Investment Company Institute and the Employee Benefit Research Institute, nearly 19 percent of workers in their 20s are not investing any of their 401(k) money in stocks, while another 16 percent have less than 60 percent in stocks.

The allocation for those in their 30s isn't much better. Thirteen percent had none of their 401(k) balances in stocks while 17.5 percent had less than 60 percent.

Being too conservative in your investments, especially at a young age, is a very costly decision.

Take a 25-year-old making $40,000 a year with $5,000 saved. And say she now invests $500 a month. By age 65, she'll have $1.4 million if she invests only 20 percent of her money in stocks and the rest in bonds and cash, according to Fidelity's new retirement planner. But if she puts 85 percent of her money in stocks, she'd have close to $2 million. That assumes average market performance.

Remedy:

Try Fidelity's easy 5-question myPlan calculator to see what a difference it will make to your nest egg if you increase your risk tolerance and/or boost your monthly savings. When filling in how much you save a month, don't forget to add in the amount of money you get in matching contributions from your employer.

And for a quick sense of the best allocation for you given your time horizon and risk tolerance, use CNNMoney.com's "Fix Your Mix" calculator to the right.

If you're not interested in allocating your own portfolio, consider investing in a target-date retirement fund, which allocates your money for you in accordance with your time horizon until retirement. (Learn more about these funds.)

If you really can't stomach risk, then you'll need to significantly increase your savings to get the same result.