Dollar-Cost Averaging Is Not a Return Booster
Investing Mindset, Part 5: the thing worth automating is not just the buy button, it is what each dollar is for.
"Dollar-cost averaging is good" and "splitting money you already have makes more money" are not the same claim. Someone investing a paycheck as it arrives cannot pull next month's paycheck forward into today. But someone who already holds a lump sum and chooses to split it into installments leaves part of that money sitting in cash while it waits. The comparison starts from a different place depending on which situation you are actually in.
What the 68% Figure Actually Answers
Vanguard compared investing a lump sum immediately against splitting it into three monthly installments, using MSCI World returns from 1976 to 2022. Under the assumptions of a 100% equity portfolio, three equal installments spaced a month apart, and no interest earned on the uninvested portion, the immediate lump-sum approach produced a larger portfolio after one year in 68% of the historical periods studied. That is a repeated comparison across past periods, not a probability of success going forward. Source
That does not mean everyone should put a lump sum into stocks all at once. For an investor who could not stomach a drop right after entering and would abandon the whole plan, splitting the investment over a limited period may be the choice that actually gets executed. Expected return and the likelihood of sticking with the plan need to be weighed together.
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| Situation | The Real Decision |
|---|---|
| Investing from a paycheck | When and how much of the new money to invest |
| Already holding a lump sum | How long to let the waiting cash sit idle |
| Money you'll need to spend soon | Whether taking equity risk with it makes sense at all |
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Buying More When It's Cheap Doesn't Automatically Make the Portfolio Better
Putting the same dollar amount in every month means buying more shares when the price is low. But that fact alone does not guarantee a profit. Keep buying a company that never recovers, and the losses can keep piling up. How you buy and the quality of what you're buying are separate questions.
"The top 10 US companies" and "a broadly diversified market" are not the same thing either. Holding several funds does not spread risk if they all overlap in the same large tech names. The SEC's investor guidance emphasizes asset allocation matched to time horizon and risk tolerance, along with diversification both across asset classes and within them. Diversification, too, does not protect against a broad market decline. Source
Three layers worth designing before automating anything:
- Spending close to daily life
- Diversified assets for long-term goals
- Additional risk you can actually afford to take
The amount in each layer depends on income stability, when the money will be spent, and how much loss you can tolerate.
Sorting cash by purpose makes the decision easier too. Money set aside for living expenses is not money that failed to get invested. On the other hand, money held indefinitely while waiting for "just a bit more of a drop" is a choice to postpone participating in the market. Cash is cash, but the standard for managing it changes depending on what it's for.
Insight Times Editorial Desk
