Because you buy a constant amount of money rather than a constant number of units, your average cost lands below the simple average of the prices you paid. The effect is mechanical, not a matter of skill.

Historically, investing a lump sum immediately outperforms spreading it out about two-thirds of the time, simply because markets rise more often than they fall. DCA's advantage is not statistical return — it is behavioural. It prevents the worst-case entry, and more importantly it keeps people invested through declines instead of freezing.

For income arriving monthly, the comparison is moot: you are averaging in because that is how the money arrives.

• On an asset in permanent decline, averaging in just buys more of a losing position.
• High per-transaction fees erode small, frequent purchases significantly.
• It cannot replace asset selection — the discipline is in the schedule, not the choice.
• Abandoning the schedule during the exact crash it was designed for defeats the entire mechanism.

Automate it so no decision is required, pick an interval that matches your income, and review the asset choice on a schedule rather than in response to price. The point of DCA is to make the decision once and then stop making it repeatedly.