Lump sum vs drip-feeding in

Suppose $60,000 arrives at once. An inheritance, a redundancy payout, a business sale, savings you have finally decided to deploy. Two options:

• Lump sum (investing it all at once). All $60,000 today.
• Dollar-cost averaging (DCA) (investing bit by bit instead of all at once). $5,000 a month for twelve months.

Note this is a question about deploying money you already hold. Investing each payday out of a salary is not DCA in this sense, it is simply investing as the money arrives, and there is no lump sum alternative to compare it to.

What the research says

The most cited study is Vanguard's, first published in 2012 under the title Dollar-Cost Averaging Just Means Taking Risk Later, which compared investing a lump sum immediately against spreading it over twelve months, across decades of history in the United States, the United Kingdom and Australia.

The result: lump sum outperformed a twelve-month phase-in in roughly two thirds of historical periods, about 68% overall, with the figure ranging from around 62% to 74% depending on the market. The average advantage was in the order of 1.2 to 2.4 percentage points, depending on the mix of shares and bonds.

The reason is not sophisticated, and it is the whole explanation:

Two honest qualifications. Roughly one third of the time lump sum lost, sometimes badly, which is not a rounding error. And this is an average across history, not a statement about the next twelve months, which nobody knows.

Why DCA still wins for a lot of real people

The statistics describe outcomes. They do not describe what a person actually does when a bad outcome arrives, and the second thing decides more results than the first. Sticking to a twelve-month schedule sounds simple on paper and is genuinely stressful to live through: every purchase after a fall feels like throwing money into a hole, every purchase after a rise feels like you should have gone all in at the start, and holding your nerve through both feelings, every month, for a year, is the actual test the statistics above do not capture.

Consider the failure case concretely. You invest $60,000 on a Monday. Six weeks later the market is down 20% and you are looking at $48,000. Nothing has been lost yet, in the sense the risk and time horizon lesson established, but the time in the market lesson explained exactly what happens next to most people: they sell, and they are absent for the recovery.

Now the same fall against a DCA schedule. You have invested $10,000 and still hold $50,000 in cash. You are down $2,000, and your next four purchases are at lower prices. The identical market event is a materially different experience, and the second version is one you will still be invested after.

The practical position

• If the sum is small relative to your existing portfolio, invest it. Phasing in $5,000 against an existing $200,000 is managing an emotion that will not arrive.
• If it is large relative to everything you own and you have never sat through a downturn, phasing in is defensible. You are paying a small expected cost for a materially higher chance of still being invested in year three.
• If you choose to phase in, decide the schedule in writing and automate it. Amount, dates, done. Otherwise it quietly becomes market timing, which the time in the market lesson dealt with.
• Keep it short. Six to twelve months. Beyond that the expected cost grows and the behavioural benefit does not.
• Do not stop the schedule because the market fell. That is the schedule working. Stopping mid-way is the worst of both approaches.

One thing both options share: neither is waiting for a better entry point. A decided, dated schedule is a plan. An undated intention to invest when things look calmer is the timing decision from the time in the market lesson, and it usually resolves into never.