Spreading an investment across several months rather than committing it at once is widely recommended. What it does is reduce exposure to a single entry price, and that comes at a cost that is rarely stated.
What the method does mechanically
A fixed sum invested at regular intervals buys more units when prices are low and fewer when prices are high, which produces an average purchase price below the average of the prices paid.
That arithmetic effect is real but modest, and it is often mistaken for the main benefit of the approach.
The larger effect is that the money not yet invested is not exposed to the market, which changes the risk profile of the whole exercise.
Why it usually lowers expected returns
Markets rise more often than they fall over long periods, so money held back is money not participating in the more likely outcome.
Comparing a gradual entry against investing a lump sum immediately, the immediate approach wins more often than not, simply because time in the market is longer.
The gap is not enormous, but it runs in a consistent direction, which is why the case for spreading entry cannot rest on expected return.
Where the genuine benefit sits
The benefit is the reduction in the worst case. Committing everything immediately before a sharp fall produces an outcome that a gradual entry would have softened.
That matters because the worst case is what causes investors to abandon a plan, and an abandoned plan performs far worse than either entry method.
Paying a small expected cost to avoid an outcome that would end the strategy is a reasonable trade, provided it is understood as insurance rather than optimisation.
Why regular contributions are a different case
Someone investing from monthly earnings is not choosing to spread anything. They are investing money as it arrives, because no lump sum exists.
The comparison does not apply there, and describing routine contributions as a timing strategy confuses a constraint with a decision.
The distinction matters because the trade-off only exists when a sum is already available and could be deployed immediately.
How the horizon changes the answer
Over long horizons the entry method fades in importance, because decades of subsequent returns dwarf the difference made by a few months of phasing.
Over short horizons the entry price matters far more, and so does the risk of needing the money before a fall has recovered.
Spreading entry over a very long period simply converts the portfolio into a cash-heavy one for years, which is a different allocation decision rather than a phasing choice.