BTW, you are correct technically that if the expected return of the investment is positive, then you maximise the expected return by putting in everything now all at once. However, maybe you want to reduce the variance. Or you want to trade off return and risk. Or you want to minimise regret.

If you put all in at a certain price, and later the market moves down, you'll regret that you didn't buy cheaper, and think you timed it badly.

If, however, you commit to a strategy of putting in say 5% per month over the next months, then a) you just automate it, and don't think about it anymore, and b) you don't really have a reference price at which you bought (sure, you can determine your actual cost basis, but who does that...) and thus avoid regret when the market tanks. Plus you reduce variance (by reducing the variance of your cost basis).

This is the original definition of DCA, but by this point most people view DCA as what everyone else in the thread is talking about.

Not a hill worth dying on.

Bad strategy? Or on average not optimal?

> DCA is something like "I have $520,000 in cash right now today sitting in checking, I'm going to buy $10,000 a week of VTSAX for the next 52 weeks" which on average is a bad strategy.

Yeah for a start if you're really planning to buy $10 K of a world fund pick one with stock options. Sell a PUT secured by the $10 K with a 7 DTE. This is already guaranteed better returns (but still a bad strategy) than this dumb way of DCAing.

But when people say they DCA what they mean is basically: "I make $10 K net per month, I spent $6 K, I keep $1 K in cash and I invest the $3 K that are left". Which is actually not a bad strategy at all.