Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Is it smart to only invest in mid- and small-cap stock equity funds in my 401(k)?
Your initial premise (mid-cap and small-cap company stocks have outperformed the market) is partially correct - they have, over many 40 yr periods, provided higher returns than large caps (or bond funds). The important thing to consider here is that risk adjusted, the returns from a diversified portfolio are far more robust - with proper asset allocation you and expect high returns and reduce your risk simultaneously.  Imagine this scenario - you decide to stick to small / mid caps for 10 - 15 yrs and move into a more diversified portfolio then. Had you made that decision during a sustained period of poor small cap performance (late 80s or the 40's) you would have lost a boatload of return, as those were periods were small / mids underperformed the market as a whole, and large caps in particular. As an example, from 1946 to 1958 large caps outperformed small every single year. If 2016 were to be the first year of a similar trend, you've done yourself a major disservice.  Since the dot com crash small /mids have outperformed for sure, pretty much every year - but that doesn't mean that they will continue to do so. The reason asset allocation exists is precisely this - over a 40 yr period, no single asset class outperforms a diversified portfolio.  If you attempt to time the market, even if you do so with a multi-decade time horizon in mind, there a good chance that you will do more poorly.