Lump sum or drip feed, and what the evidence says

The points below about lump sum investing are ordered by how much difference they make, not by how often they get repeated.

The arithmetic favours going in

Markets have risen more often than they have fallen over most periods, so money held back is more often out of a rising market than a falling one. Studies across long histories have generally found immediate investment beating phased entry in a majority of periods. That is a statement about averages, not about any specific twelve months.

Phasing buys something real

Spreading entry reduces the worst case of investing everything immediately before a sharp fall. It also reduces the emotional cost of that scenario, which is not a trivial consideration. What it does not do is raise expected return; it lowers both the best and worst outcomes.

The behavioural question decides it

If a large immediate fall would cause you to sell, phasing is worth its expected cost. If it would not, the evidence supports investing sooner. Answering that honestly is more useful than any statistic, because a plan abandoned mid-way performs worse than either approach.

Phasing has a deadline

Where phasing is chosen, a defined schedule — a fixed amount monthly over a set period — prevents it becoming indefinite hesitation. Cash held indefinitely while waiting for clarity is the outcome that consistently underperforms. Writing the schedule down converts it from a feeling into a plan.

Some of this will suit you and some will not, and that is the point.

Regular contributions are a different thing

Investing monthly from income is not phased entry; it is simply investing as money arrives. The lump sum question only applies to money you already hold. Confusing the two produces unnecessary anxiety about a decision that is not being made.

Everything above, in order of what to do first

1. The arithmetic favours going in. Markets have risen more often than they have fallen over most periods, so money held back is more often out of a rising market than a falling one.
2. Phasing buys something real. Spreading entry reduces the worst case of investing everything immediately before a sharp fall.
3. The behavioural question decides it. If a large immediate fall would cause you to sell, phasing is worth its expected cost.
4. Phasing has a deadline. Where phasing is chosen, a defined schedule — a fixed amount monthly over a set period — prevents it becoming indefinite hesitation.
5. Regular contributions are a different thing. Investing monthly from income is not phased entry; it is simply investing as money arrives.

The takeaway

Ask whether you would sell after a thirty per cent fall. Answer that, and the question resolves itself.

Small and repeatable beats ambitious and abandoned, almost every time.