Dollar-Cost Averaging

Dollar-cost averaging is investing a fixed dollar amount on a fixed schedule regardless of what the market is doing: $500 on the first of the month, every month, through rallies and drawdowns alike. Fixed amounts buy more shares when prices are low and fewer when they are high, which mechanically lowers your average cost per share below the average price. In the US, it is already running by default: every automatic 401(k) contribution and scheduled brokerage transfer is dollar-cost averaging, whether or not the investor names it. The strategy removes the timing decision, which is the decision investors are worst at.

The honest caveat is that dollar-cost averaging is not the mathematically optimal way to invest a lump sum. Vanguard's research found that investing a lump sum immediately beat a 12-month averaging schedule roughly two-thirds of the time across US markets, because markets rise more often than they fall. Averaging wins the other third and, more importantly, wins the regret test: an averaged lump sum never has to watch the entire balance fall 30% in the month after it lands. The strategy is insurance as much as it is investing.

The mechanism is the per-lot cost basis. Take $1,000 invested monthly at three prices: at $50 it buys 20 shares, at $80 it buys 12.5, and at $40 it buys 25. That is 57.5 shares for $3,000, an average cost of about $52.17 per share against an average price of $56.67. The averaging worked because the fixed dollar amount bought its largest parcel at the cheapest price, which is the entire trick: dollars, not shares, are held constant.

In practice, the schedule runs itself. The 401(k) contribution lands every payday, buys fund shares at that period's price, and repeats; the brokerage auto-invest does the same for an ETF like a broad S&P 500 fund, and the dividend reinvestment plan compounds it further. Set it up once, tie it to the pay cycle, and the average cost per share takes care of itself through whatever the market does next. The investor's only job is to never pause the schedule, because the pause is the timing decision sneaking back in. The dollar-cost averaging arithmetic favors the disciplined over the clever, every time it is run.

Dollar-cost averaging is the default machinery of American investing: the 401(k) runs it every paycheck, the Roth IRA contribution runs it, and every scheduled brokerage transfer runs it. Its value is behavioral as much as mathematical, because the strategy removes the buy-timing decision that investors reliably get wrong. The research is clear that lump sums win on arithmetic roughly two-thirds of the time, and equally clear that most investors cannot watch a full balance fall 30% a month after deploying it.

I watched a colleague named Brent (yes, that Brent) pause his 401(k) contributions through 2022 to 'wait for the bottom'. He missed the cheapest shares in a decade and re-entered at higher prices once the news improved. The bottom announced itself afterwards, as bottoms do.

The highest-leverage thing most Americans can do is automate the schedule at the pay-cycle frequency, leave it running through drawdowns, and price any lump-sum averaging decision honestly before making it. Run the numbers through the dca calculator or the dont-delay-savings calculator before pausing anything.