Interview of David Swensen, Chief Investment Officer of Yale Endowment.

I quote some here:

Y: Explain this idea of asset allocation, please.

S: Asset allocation is the tool that you use to determine the risk and return characteristics of your portfolio. It’s overwhelmingly important in terms of the results you achieve. In fact, studies show that asset allocation is responsible for more than 100 percent of the positive returns generated by investors.

Y: How can that be?

S: It’s because the other two factors, security selection and market timing, are a net negative. That’s not surprising. They’re what economists would call zero-sum games. If somebody wins by buying Microsoft, then there has to be a loser on the other side who sold Microsoft. If it were free to trade Microsoft, the amount by which the winner wins would equal the amount by which the loser loses. But it’s not free. It costs money. It costs money in the form of market impact and commissions if you’re trading for your own account, and it costs money in terms of paying fancy fees if you are relying upon an investment advisor or mutual fund to make these security-specific decisions. For the community as a whole, all those fees are a drag on returns.

That’s why the most sensible approach is to come up with specific asset allocation targets that you can implement with low-cost, passively managed index funds and rebalance regularly. You’ll end up beating the overwhelming majority of participants in the financial markets.