EFF/KRF: There is no single right answer to this question. If there were one answer, it would have to be the market portfolio of domestic, international, and emerging stocks, since that is the only portfolio that can be held by everyone. Different tastes and circumstances, however, push investors away from the market portfolio and the optimal deviations vary across investors. For example, perhaps because of exchange rate uncertainty, people tend to overweight their domestic market. Similarly, some investors are happy to increase their expected return by tilting toward small stocks, while others prefer to reduce their risk by tilting toward large caps. It is important to diversify, but there is no single optimal mix.
Eugene Fama and Ken French are members of the Board of Directors for and provide consulting services to Dimensional Fund Advisors LP.