Investing a fixed sum at regular intervals is widely recommended, often with claims that do not survive examination. It does something real, but not what is usually advertised.
The mechanical effect on average price
A fixed amount buys more units when prices are low and fewer when prices are high, because the quantity purchased varies inversely with price.
The result is that the average price paid per unit falls below the average of the prices at which purchases occurred. That is arithmetic, not market insight.
The effect is real but modest, and it grows with volatility. In a steadily rising market it produces very little.
It is a risk decision, not a return strategy
Spreading a lump sum over time keeps part of the money out of the market while it waits, and markets rise more often than they fall over long horizons.
That means staged entry has historically produced a lower expected outcome than investing the full amount immediately, in exchange for a narrower range of outcomes.
The trade is protection against the worst timing at the cost of the average result. Whether that is worthwhile depends on the investor, not on the market.
Regular contribution is a different thing entirely
Someone investing part of each paycheque is not choosing to stage entry. They have no lump sum, and the pattern simply reflects when income arrives.
Conflating the two causes confusion, because the criticisms of staged entry do not apply to a person who is investing money as it becomes available.
For that investor the discipline of automatic contribution is the substantive benefit, and the averaging effect is incidental.
The behavioural argument is the strongest one
The most common way investors damage results is by acting on discomfort, buying after strong periods and selling after weak ones.
A fixed schedule removes the decision. Contributions continue during falls, which is precisely when discretionary investors tend to stop.
A strategy that is slightly worse in theory but actually followed will beat an optimal one that is abandoned during a decline.
Where the approach can mislead
Averaging does not prevent loss. An investor buying steadily into an asset that never recovers simply accumulates more of something that has fallen.
The method also says nothing about what is being bought, and a disciplined schedule applied to a poorly diversified holding remains poorly diversified.
The choice of asset and the level of cost do far more to determine the outcome than the timing pattern of contributions.