“I just got a $30,000 inheritance and I want to put it in index funds. Should I invest it all now, or spread it out over the next year so I don’t buy right before a crash?”
It is one of the most common questions new investors ask, and the honest answer has two parts. Mathematically, investing the whole amount at once usually comes out ahead. Behaviorally, spreading it out can be the difference between investing and never getting around to it. Which part matters more depends on you.
In short
If you get paid every two weeks and invest part of each paycheck, you are already dollar-cost averaging, and there is nothing to decide: that money goes in as soon as you have it. The real choice only appears when you hold a lump sum. Historically, investing it right away has beaten spreading it out about two times in three, because markets rise more often than they fall and cash waiting on the sidelines misses that growth. Spreading it out costs some expected return in exchange for less regret if prices drop soon after you start. That trade can be worth making if a bad first month would make you quit.
What dollar-cost averaging actually does
Dollar-cost averaging means investing a fixed dollar amount on a fixed schedule regardless of price. Because the dollar amount stays the same, you automatically buy more shares when prices are low and fewer when they are high. The SEC’s investor education site describes it as a way to reduce the impact of volatility on your purchases.
Here is a hypothetical example. Say you invest $3,000 a month for four months in a fund whose price moves like this:
| Month | Amount invested | Share price | Shares bought |
|---|---|---|---|
| 1 | $3,000 | $50 | 60 |
| 2 | $3,000 | $40 | 75 |
| 3 | $3,000 | $25 | 120 |
| 4 | $3,000 | $40 | 75 |
| Total | $12,000 | Average price $38.75 | 330 |
You end with 330 shares at an average cost of about $36.36, below the $38.75 average of the four prices, because the cheap month bought the most shares. If you had invested all $12,000 in month one at $50, you would have 240 shares. In this particular falling-then-recovering market, averaging in clearly won.
But notice what made that work: the price dropped after the first purchase. Flip the scenario so the price rises steadily from $50 to $70, and the lump sum wins easily, because every later purchase buys fewer shares at higher prices.
What the research says about lump sums
Since markets rise more often than they fall over most 12-month windows, the scenario where averaging wins is the less common one. Vanguard’s 2023 paper “Cost averaging: Invest now or temporarily hold your cash?” tested this with global stock market data from 1976 to 2022. Investing a lump sum immediately beat splitting it into three monthly installments about 68% of the time, measured after one year. The longer you stretch out the averaging period, the more often the lump sum wins, since more money sits in cash for longer.
The same research found something just as useful: cost averaging still beat leaving the money in cash about 69% of the time. In other words, the worst option is usually not the slow one. It is the one where you keep waiting for a better entry point that never feels safe enough.
Why people still choose to average in
If the math leans toward lump sums, why does anyone spread it out? Because the outcomes are lopsided emotionally, not just financially.
- Regret is asymmetric. Investing $30,000 the week before a 20% drop means watching $6,000 disappear on paper. Many people find that harder to live with than missing some gains.
- Bad timing can trigger worse decisions. An investor who panics and sells after an early loss locks it in. If averaging keeps you invested, it has done its job. Our guide on what to do when the market drops covers that risk.
- It creates a plan. “Invest $5,000 on the first of each month for six months” is concrete and can be automated. “Wait for a dip” is not a plan.
How to average in without overthinking it
If you go this route, keep the window short and the schedule fixed. Three to six months is common; stretching a lump sum across several years leaves too much in cash for too long. Pick the dates in advance, set up automatic purchases if your brokerage allows it, and park the uninvested money in a high-yield savings account or Treasury bills so it earns something while it waits. Do not change the schedule based on headlines. The moment you start skipping months because the market “looks high,” you have switched from averaging to market timing.
When the question doesn’t apply
A few situations make the debate irrelevant:
- Paycheck investing. Regular 401(k) or IRA contributions already work this way, and that is a good thing. Small, automatic amounts are exactly how most people build wealth, as our article on investing $100 a month shows.
- Money you need soon. If the $30,000 is earmarked for a house down payment in two years, neither strategy is appropriate; short-term money generally belongs in cash-like accounts, not stocks.
- High-interest debt. If you carry credit card balances at 20% or more, paying them off is a guaranteed return that neither approach can match.
The practical answer
If you have a lump sum, a long time horizon, and the stomach to watch it drop without selling, investing it all at once has historically given you the best odds. If you are honestly unsure how you would react to an early loss, splitting it into three to six equal monthly purchases is a reasonable middle path that costs relatively little in expected return. Either is far better than leaving the money in a checking account waiting for the perfect moment. And for ongoing savings from each paycheck, the answer is simple: invest it as it arrives, on autopilot, and stop checking prices.
