No diversification only serves to lower risk- if done correctly. To increase risk, concentrate your position. This could increase or decrease return greatly over time from a diversified portfolio. Diversified portfolios spread out your assets and make long term investing outcomes much more narrow.

Diversification decreases risk for a given return. Concentration increases risk. Don't confuse holding a collection of funds with diversification. Adding international to US increases diversification because your investment is spread among more stocks. Adding US small cap to a US total market index is concentrating in small cap because total US already includes small cap.

It is the opposite - do DECREASE risk. The person who can do the most damage to your investments is YOU. If diversifying into a solid mixture of stocks/bonds/REITS etc helps to decrease volatility - it will prevent you from throwing in the towel and selling your stocks if the market takes a 30-40-50% dive. A proper asset allocation will allow you to sleep well at night no matter what the market is doing.