Kaplan: Long-term investing works when you keep your money in the market for long periods of time. When you're doing dollar cost averaging, you're not keeping your money in the market over the full period of time. You are keeping much of your money out of the market for much of the time. So, you're not getting the full benefit of long-term investing. Furthermore, when you're dollar cost averaging, you're basically placing a bet. You're placing a bet that the market will be going down shortly after you get the money and that eventually the market will come back up towards the end of the period as you're putting more money into the market.