Drip-feeding money in is a decision about regret, not about returns

Everything below about phasing money into the market comes from what actually happens rather than from what is supposed to.

The arithmetic of delay

If a market has a positive expected return, money held out of it earns less on average than money in it. Investing a lump sum in stages therefore has a lower expected outcome than investing it at once, which studies across many markets and periods have generally found. The margin is not enormous, but the direction is consistent and follows from the assumption that markets rise more often than they fall.

Anyone recommending phasing on the grounds that it produces better returns has the arithmetic backwards.

What phasing actually buys

It narrows the range of outcomes, in particular the outcome where you invest everything the day before a substantial fall. That outcome is not the most likely one, but it is the one most likely to make someone sell and never return.

Paying a small amount of expected return to reduce the chance of abandoning the plan entirely can be entirely rational. The honest framing is that phasing is insurance against your own reaction, not a return-enhancing technique.

Choose the period deliberately

Phasing over three to six months captures most of the reduction in worst-case risk while limiting the expected cost. Phasing over three years leaves most of the money uninvested for most of the period, which is a large drag if the horizon is long. Committing to the schedule in advance matters, because a schedule abandoned when markets fall is the opposite of the intended behaviour.

Automating the instalments removes the monthly decision that phasing otherwise creates.

Regular investing from income is not the same thing

Someone investing part of each month's salary has no lump sum to deploy, so the comparison does not arise. They are investing each amount as soon as it exists, which is the lump-sum approach applied to a stream. The averaging effect that follows is a by-product of when they are paid, not a strategy.

Confusing the two leads people to think monthly investing is a technique rather than a description of their cashflow.

The horizon question comes first

None of this matters if the money is needed within a few years, in which case market exposure may be inappropriate regardless of how it is phased. Phasing does not make a short horizon safe; it spreads the entry point and leaves the exit date unchanged.

For most households, the question of whether to invest at all precedes the question of how to enter, and it depends on capacity for loss. That is a matter for regulated advice rather than a general rule, particularly for a large sum.

The right answer depends on your tax situation, which this cannot see.

Waiting for a better moment is a third thing

Holding cash while waiting for a fall is market timing, and it requires being right twice — about when to leave and when to return. Evidence that this can be done consistently is weak, and the cost of being wrong is missing periods when markets rise sharply.

In numbers, a large share of long-run returns has historically come from a small number of strong days, which are impossible to identify in advance. Phasing on a fixed schedule is a defined process; waiting for the right moment is an open-ended one with no exit rule.

The takeaway

Phasing is insurance against your own reaction. Priced honestly, it is sometimes worth buying and never a way to earn more.

The decision is rarely about picking the best option — it is about avoiding the expensive one.