Dollar-Cost Averaging vs. Lump-Sum Investing: What the Data Actually Shows

You just got a $20,000 bonus, an inheritance, or a rollover check. Do you invest all of it today, or spread it out over the next year?

What Vanguard’s Research Found

Vanguard’s often-cited study analyzed 1,021 rolling 12-month periods across U.S., U.K., and Australian markets from 1926 to 2015. Investing a lump sum immediately outperformed spreading it out via dollar-cost averaging in about 68% of those periods, for a balanced 60/40 stock-bond portfolio. Stretching the dollar-cost-averaging period from 12 months to 36 months pushed lump sum’s win rate even higher, to around 90%.

Why Lump Sum Wins More Often Than Not

Markets rise more years than they fall — U.S. stocks have posted positive returns in roughly 70-75% of rolling 12-month periods historically. Every month new money sits in cash while being dollar-cost averaged in in is a month it’s earning a money-market return instead of a market return, and that drag adds up across the majority of periods. The same fee and return math behind choosing an index fund over an active fund applies regardless of which timing approach is used.

Where Dollar-Cost Averaging Actually Wins

The roughly one-third of periods where dollar-cost averaging comes out ahead tend to cluster around market downturns — buying in stages during a falling market means the average cost basis ends up lower than committing everything at the top. If a lump sum happens to land right before a downturn, the psychological cost of watching it drop immediately is real, even if the long-run math favors lump-sum investing on average.

The Practical Middle Ground

Dollar-cost averaging is really a tool for managing regret and volatility risk, not a strategy that beats lump-sum investing on expected return. Someone who can’t stomach investing a large sum right before a possible drop can split the difference — investing over three to six months rather than 36 — capturing most of the statistical advantage of lump-sum investing while reducing the single-point-in-time risk.

What This Doesn’t Apply To

Ongoing 401(k) or IRA contributions from a paycheck are dollar-cost averaging by necessity, not by choice — this comparison is specifically about what to do with a one-time sum already in hand, not about whether to keep contributing normally from income.

Written by Samuel, Certified Public Accountant

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