Lump sum vs dollar-cost averaging

You have a meaningful sum to invest — an inheritance, a bonus, proceeds from a sale. Put it all in at once, or spread it over the next several months? The internet is confident in both directions. The research is not ambiguous, but it is narrower than either side admits.

What Vanguard found

The most cited study on this compared investing a lump sum immediately against spreading it over twelve monthly instalments, across rolling ten-year periods in three markets.

Across all three markets, lump-sum investing produced a higher ending balance in roughly two-thirds of the periods studied. The portfolio modelled was 60% stocks and 40% bonds, and the result held regardless of the exact asset mix.

Why lump sum usually wins

The reason is almost boringly simple, and the researchers state it plainly: over the full span in each market, the average returns of stocks and bonds exceeded the return on cash. Money sitting in cash waiting to be deployed is money earning the lowest-returning asset in the portfolio. Spread your purchases over twelve months and, on average, you hold roughly half the sum in cash for half a year.

That also tells you when the logic weakens. In a period when cash yields 5% rather than nothing, the cost of waiting shrinks considerably. The gap between the two strategies is not fixed — it moves with the spread between expected portfolio returns and the risk-free rate.

The other third

One period in three, averaging in won. Those are the periods where the market fell after the moment you would have invested the lump sum, so buying gradually meant buying some of it cheaper.

This matters more than the headline suggests, because the distribution is not symmetric. The worst outcome for lump-sum investing — putting everything in immediately before a severe decline — is considerably worse than the worst outcome for averaging. Lump sum has the better average and the fatter left tail.

So which should you choose?

The expected-value answer is lump sum. But the expected-value answer assumes something about you that may not be true: that you will hold through a 30% decline three months after investing without selling.

If you would not — if a bad start would make you capitulate at the bottom, or simply lose sleep for a year — then averaging is not the irrational choice. It is paying a small, quantifiable premium (about 2% of ending value, on average) for a smaller worst case and a plan you will actually stick to. A strategy you abandon returns nothing.

The honest framing is that this is a question about your temperament with a price tag attached, not a question about mathematics with a wrong answer.

Two things that are not this decision

Regular contributions from salary are not dollar-cost averaging in this sense. If you invest each month because that is when you get paid, you are not choosing to delay — there is no lump sum sitting in cash. The research above does not apply to you, and there is nothing to optimise.

Neither strategy is market timing. Both commit to being fully invested by a fixed date decided in advance. Waiting for a dip before starting is a different thing entirely, and it is the one approach the evidence is genuinely unkind to.

Source: Vanguard, Dollar-cost averaging just means taking risk later. Figures are historical averages over the periods stated and are not predictions.