Invest it all at once, or a bit at a time?

Stock markets spend most of their time going up: the falls are sharper but rarer. If you spread your entry over twelve months, for eleven of them part of your money sits idle — and idle money earns nothing while the market climbs.

Across long periods and many different starting points, going in all at once beats phasing in about two times out of three. That isn't an opinion, it's arithmetic: the more time your money spends invested, the more of the work it does.

The one case in three where you lose is the one where you invest just before a sharp fall. It doesn't only cost money: it puts you in front of a portfolio in the red in the first months — exactly when you have the least reason to be patient and the most temptation to sell.

And whoever sells during a fall does not get the recovery: they turn a temporary loss into a permanent one. That behavioural mistake almost always costs more than the statistical gap between the two methods.

The right question isn't "which returns more", it's "with which of the two can I leave it untouched for ten years". If a 30% fall in the first year would make you sell, phasing in isn't the inferior choice: it's the only one that works for you.

A common middle road: half now, half spread over six or twelve months. You give up part of the statistical edge and buy in return the odds of staying seated when the first bad week arrives.