Morningstar has taken an in-depth look at target-date retirement funds in a new research report.

Target-date retirement funds allow you to take a "set it and forget it" approach to investing. You simply identify your expected year of retirement and then put all of your 401(k) or Roth contributions into the matching target-date retirement fund. Fund managers then steadily decrease the level of risk in your portfolio as you approach your retirement date.

Morningstar's two favorite companies for target-date retirement funds are Vanguard and T. Rowe Price. That info jibes very nicely with Clark's favorite picks on his investing guide. One area where Morningstar and Clark diverge is on Fidelity. Clark likes Fidelity for target-date retirement funds, while Morningstar does not.

Remember, all of the companies we've discussed so far sell their target-date retirement fund commission free.

But what if you absolutely want to pay someone to pick a target retirement fund for you? Morningstar suggests you're likely to have a good experience with American Funds, one of the largest commission houses. Independent studies have in the past confirmed this.