You're either the kind of person who has an investment thesis that you're executing on, trying to time the market for optimal returns, or you're the kind of person who should cost average into the stock. Based on the question the GP asked, they're in that second group.
If you make the random walk assumption then DCA doesn't improve expected returns. Those who expect higher returns are (perhaps unknowingly) relying on unjustified cyclic patterns in prices and trying to exploit that inefficiency.
DCA can reduce risk in theory, but only if you consider USD a risk-free asset. If you really want to minimize risk you should buy real estate funds, commodity funds, etc. based on your expected future consumption.
