"Consider a portfolio with a mix of stock types, such as smaller companies, international companies, emerging markets, etc. for your equity allocation. Diversification is the only free lunch when it comes to investing. Spreading your bets across many different investments may mean you won’t have top returns in any year but will give you the best chance to have strong returns over the long haul."
Quoted from:What stock-market return should your financial plan assume? By David Blanchett, 3/31/16. David Blanchett is the head of retirement research for Morningstar Investment Management.