I agree, that on average, just doing a lump sum is better, but the difference really isn't that big. I am incredibly comfortable with the idea that if I do DCA over a year, on average I'll be a couple of percentage points less, but I will reduce my variance, and hence my chance of getting a really bad income.
I also think it depends on how long you've been investing. It is incredibly demoralizing for a new investor, to put in a lot of money and then see it fall. For new investors, I think psychologically its very good for them to start to invest in dribs and drabs, so that if the market falls after their first investment, it doesn't hurt them emotionally so much.
I'd probably support a rule, something like, if you're going to increase your stock market investment by 20% or less, just do it. Otherwise, use DCA over the next months, for no longer than a year.
Of course, this is all based on my experience, way back in 2000 I had about $800 in the share market, and its gradually increased, by about 60% a year, since then. So I guess, in supporting gradualism, I'm just saying what I did, and its outcome, seem good.
