Investing gradually rather than all at once is widely described as reducing risk. It does change the distribution of outcomes, but it is a trade rather than an improvement.
The mechanism is mechanical, not predictive
A fixed dollar amount buys more units when prices are low and fewer when prices are high, because the amount is constant and the price is not.
Over a series of purchases, this produces an average cost lower than the average of the prices paid, which is a property of the arithmetic rather than a forecast.
No judgment about the market is involved, which is what makes the approach usable by someone with no view on where prices are going.
The comparison depends on the starting position
For someone investing from each paycheck, gradual investing is not a choice. The money does not exist yet, and there is nothing else to compare it against.
The genuine comparison arises when a lump sum already exists, and the question is whether to deploy it at once or spread it over months.
Those are different situations that share a name, and conflating them is the source of most confusion about whether the approach helps.
Spreading a lump sum keeps cash uninvested
Deploying gradually means part of the money sits in cash for the duration, exposed to whatever cash earns rather than to the intended assets.
If the assets rise over the period, the delayed portion missed that rise. If they fall, the delayed portion avoided it.
Which happens is unknowable in advance, so the approach cannot be described as reducing risk generally. It substitutes one exposure for another.
The real benefit is behavioral
What gradual investing reliably reduces is the consequence of a single bad entry point, and with it the regret that causes people to abandon a plan.
An investor who would sell after an immediate decline is worse off having invested everything at once, regardless of what the arithmetic says.
Framing it as managing one's own likely behavior is more accurate than framing it as managing market risk, and it is the more useful framing.
Costs and mechanics still apply
More frequent purchases mean more transactions, which matters where commissions or spreads exist and matters less where they do not.
Automatic scheduled investing also removes the recurring decision, which is why employer retirement plans are structured this way by default.
None of this speaks to what should be bought, which is a separate question governed by circumstances, time horizon, and factors specific to each household.