The main assumption in equity investing is that a higher degree of volatility (or risk) means a higher potential (or expected) return on investment. Conversely, investors who take on a low degree of risk have a low expection for return.
You can create optimal portfolios in USA market or optimize your existing portfolio in one of two ways: 1)
For any level of risk, select the one which has the highest expected return. 2)
For any expected return, select the one which has the lowest volatility.