Investing · Reference

Dollar-Cost Averaging: The Case For and the Case Against

Buying at fixed intervals lowers your average cost per share. It also, on average, lowers your return — both are true.

Dollar-cost averaging means investing a fixed amount at fixed intervals rather than a fixed number of shares or one lump sum. It is widely recommended and the reasoning given for it is usually half right, so it is worth separating the parts that hold from the parts that do not.

The arithmetic that does hold

A fixed sum buys more shares when the price is low and fewer when it is high. That is not a psychological claim, it is division, and it produces an average cost below the average price.

Month	Price	$300 buys
1	$30	10.00 shares
2	$20	15.00 shares
3	$25	12.00 shares

$900 has bought 37 shares, an average cost of $24.32. The average of the three prices is $25.00. The gap is real and it comes from weighting more dollars toward lower prices — a harmonic mean rather than an arithmetic one.

The case against, which is rarely stated

Compare averaging in against investing the same total immediately. Markets rise more often than they fall, so money held back to be invested later spends that time out of the market. Studies of historical data consistently find lump-sum investing ahead roughly two-thirds of the time, by a couple of percentage points on average.

Dollar-cost averaging is therefore not a way to earn more. It is a way to reduce the consequence of bad timing at a small expected cost — insurance, priced in expected return. That is a legitimate thing to buy; it is just not what the usual pitch says.

The distinction that matters

Two quite different things share the name.

Investing money as you earn it. Contributing every payday is not a strategy at all, it is the only option — you cannot lump-sum money you do not have yet. The comparison with lump-sum investing does not apply, and this is what most people are doing.
Holding a lump sum back to spread it out. This is a genuine choice, and it is the one the research is about. Here you are accepting a lower expected return for a lower chance of a bad entry point.

Most arguments about dollar-cost averaging are two people discussing different ones of these.

When holding back is defensible

The expected-return argument assumes you will hold either way. If a 20% fall immediately after investing everything would make you sell, then the comparison is not lump sum against averaging — it is averaging against abandoning the plan, and averaging wins easily. The behavioural case is stronger than the mathematical one and worth taking seriously on its own terms.

The other clean case is genuine uncertainty about the horizon: money that might be needed soon should not be committed all at once regardless of what the averages say.

What it does not do

It does not protect against a sustained decline — buying every month on the way down leaves you holding more of something worth less. It does not reduce volatility once you are fully invested; it only shortens the period during which you are exposed to a single entry price. And it is no substitute for the allocation decision, which determines far more of the outcome than the schedule of entry ever will.

Reference material, not investment advice.