Investing a lump sum gradually rather than all at once is widely recommended and widely misunderstood. The method changes the distribution of outcomes rather than improving the average one.
What the method actually does
Dollar cost averaging takes a fixed sum and invests it in equal instalments across a set period, buying more units when prices are low and fewer when prices are high.
The resulting average purchase price is below the average of the prices paid attention to along the way. That arithmetic is real and often cited as the main benefit.
It is not the whole picture, because the money awaiting investment is not earning the return of the asset being bought. That opportunity cost sits on the other side.
Expected return favors investing immediately
Markets have historically risen more often than they have fallen over most holding periods. Any strategy that keeps money out of the market for longer gives up some of that drift.
Averaging in therefore has a lower expected return than investing the full amount at the start. The shortfall grows with the length of the phasing period.
This is a statement about averages across many outcomes, not a prediction about any particular year. In a falling market the phased approach comes out ahead.
The benefit is in the spread of outcomes
Investing everything at once concentrates the entry point into a single day. The result depends heavily on whether that day happened to sit near a peak.
Spreading the purchase over several months averages across many entry points, which narrows the range of possible results. The best outcomes get worse and the worst get better.
For someone who cannot tolerate the worst case, that narrowing is worth paying for. The payment is the modest reduction in expected return described above.
Behavior is a legitimate reason
An investor who commits everything just before a sharp decline may abandon the plan entirely and sell near the bottom, converting a paper loss into a permanent one.
Phasing reduces that risk by keeping the early stakes small enough to stay with. A strategy that is actually followed beats a better strategy that is abandoned.
This is a real justification rather than a consolation. Portfolio outcomes depend as much on holding through declines as on the mathematics of the initial purchase.
Regular contributions are a different case entirely
Money arriving from a salary each month is invested as it appears. There is no lump sum being deliberately held back, so the opportunity cost argument does not apply.
This pattern is often called dollar cost averaging, but it is simply investing income when it arrives. The comparison with immediate investment of a windfall is a separate question.
Conflating the two leads people to believe that phasing an inheritance carries the same logic as a payroll deduction. The mechanics and the trade-offs are not the same.