What the research shows
Vanguard compared investing a sum at once with splitting it into three equal parts invested a month apart, using the MSCI World index from 1976 to 2022. The lump sum came out ahead 68% of the time. Spreading the money out still beat leaving it in cash 69% of the time.[1]
Why the lump sum usually wins
Markets have risen in more periods than they have fallen. Money that waits on the sideline therefore earns less on average than money that is already invested. Waiting for the right moment is a decision to stay in cash a little longer.
Why people still spread it out
About one time in three, the lump sum did worse. Spreading the purchases over a few months lowers the risk of investing everything just before a fall, at the price of a lower expected result. If a sharp drop right after investing would make you sell, spreading it out can be the better choice for you.
Monthly saving from your salary is something else
Investing part of each salary is not delaying a lump sum. It is investing each amount as soon as you have it, which is exactly what the research favours.
Common questions
What if the market falls right after I invest?
It can happen, and it did in roughly a third of the periods in the Vanguard study. For money you will not need for many years, a fall early on matters less than the years of growth that follow. Past performance does not predict future returns.
Over how long should I spread a lump sum?
The study used three months. The longer the schedule, the longer part of the money stays in cash. If you choose to spread it out, decide the dates in advance and stick to them.
Sources & further reading
We cite independent authorities so you can verify everything yourself. Last reviewed 15 Sept 2026.