Drip-Feeding Versus Investing a Lump Sum: What the Numbers Say

You have a meaningful sum to invest: an inheritance, a bonus, the proceeds of a sale. The question is whether to put it in today or spread it over the next year. The evidence is clearer than the argument suggests.

What the evidence shows

Across long historical periods and most major markets, investing a lump sum immediately beats spreading it out roughly two thirds of the time. The reason is unglamorous: markets rise more often than they fall, so money waiting on the sidelines is more often missing gains than avoiding losses.

The margin is real but not enormous, and it comes with a wider range of outcomes. Investing everything the month before a sharp fall is a genuinely worse experience than having averaged in, even if the average case favours going in at once.

When drip-feeding is the better choice

• The sum is large relative to your existing savings. Doubling your invested wealth in a single transaction is a different psychological event from adding a few per cent to it.
• You have never held through a fall. First-time investors who invest everything and immediately watch it drop often sell, which converts a temporary decline into a permanent loss.
• You would genuinely lose sleep. This is not a soft consideration. An investor who abandons a good plan under stress does worse than one who followed a slightly suboptimal plan calmly.

When to just invest it

• You are already invested and adding to a portfolio you have held through at least one bad year.
• The horizon is long. Over twenty years the entry point matters far less than the time spent in the market.
• The alternative is indecision. Plenty of "I will drip it in" money is still sitting in cash three years later, which is the worst outcome of the three.

If you do spread it out

Make it mechanical. Choose the amount and the dates in advance, automate the transfers, and do not adjust them because of the news. A schedule you change in response to headlines is not drip-feeding; it is market timing with extra steps.

Keep the period short. Six to twelve months captures nearly all the psychological benefit. Spreading a lump sum over three years mostly means holding cash for three years, and the cost of that compounds.

The honest summary

If you want the highest expected return, invest it now. If you want the highest chance of still being invested in five years, and you know yourself to be nervous, spread it over six months and stop reading about it. Both are defensible. What is not defensible is leaving the decision open indefinitely, because that is a choice to hold cash without ever admitting you made it.