Dollar-Cost Averaging (DCA)

Dollar-cost averaging is a commitment device dressed up as an investment strategy: a fixed dollar amount goes into the market on a fixed schedule, and the schedule, not your mood or the headlines, decides when you buy. Because the amount is constant while prices move, the math tilts gently in your favor--each $500 buys more shares in a dip than at a peak. But the deeper purpose is to remove timing decisions entirely, since the investor who waits for the right moment usually ends up waiting through the gains.

It helps to separate two situations that both get called DCA. The first is investing income as it arrives: contributing from every paycheck into a 401(k) or IRA. Here there is no alternative to compare against, since the money did not exist earlier, and automatic scheduled investing is simply how sensible investing works. The second is holding a lump sum--an inheritance, a bonus, a home-sale windfall--and choosing between investing it all today or feeding it in over months. Only the second is a genuine strategy decision.

For that decision, the honest arithmetic favors the lump sum: markets rise more often than they fall, so money spread over months sits partly in cash during what are, more often than not, rising markets. Studies of long historical periods consistently find lump-sum investing beats a multi-month DCA schedule most of the time. What DCA buys instead is protection from the scenario people fear most, investing everything the week before a crash, and protection from the paralysis that fear causes. An investor who would otherwise leave the money in cash for years while waiting for clarity is far better off with a six- or twelve-month DCA schedule they will actually follow than with an optimal plan they will not.

A hypothetical example of the mechanics. Theo invests $500 a month into a stock fund for three months. The share price is $10 in January, $8 in February, and $12.50 in March. His purchases buy 50 shares, then 62.5, then 40, for a total of 152.5 shares.

He spent $1,500 for 152.5 shares, an average cost of about $9.84 per share, while the simple average of the three prices was about $10.17. The fixed dollar amount did that automatically, loading up when shares were cheap in February and easing off when they were dear in March. No forecast was involved, and none was needed. Over three months the edge is pennies; the durable benefit is that Theo bought in February at all, when the falling price would have scared off many investors deciding month to month.

Pros

• Eliminates market-timing decisions, which investors get wrong far more reliably than they get right.
• Automates the habit, so investing continues through scary headlines and busy months alike.
• Mechanically produces an average cost per share below the average price over the period.
• Softens the regret and the consequences of starting right before a downturn.

Cons

• For a lump sum already in hand, it usually trails investing everything immediately, since markets rise more often than they fall.
• Money waiting its turn in the schedule earns cash returns while it waits, a drag in rising markets.
• Can become a security blanket: some investors stretch the schedule so long that it amounts to staying in cash.