The Behaviour Gap

The third thief

There are three forces quietly removing money from your portfolio. Two of them are visible and much discussed. Inflation erodes what your money buys, at around 2% a year. Fees come straight off your return, typically a fraction of a percent to well over one percent. Both matter, and both have a calculator here: inflation and fees.

The third thief is the one nobody puts on a statement, because it does not appear on one: your own behaviour. It is the return you give up by acting on emotion — buying after the crowd, selling into fear, sitting in cash waiting for certainty. And it is usually the biggest of the three.

The number that should stop you

DALBAR has tracked this for decades by comparing what funds returned with what the investors in them actually earned. In 2024 the S&P 500 returned 25.05%. The average equity investor earned 16.54% — a gap of 8.48 percentage points, in a year the market went straight up. The money was not lost to a bad market or to fees. It was lost to timing.

A single year is noise. The point is what that behaviour does when it repeats. Pick a reaction, press start, and watch the two of you run through 30 real years — dot-com, 2008, 2022. The shaded gap that opens up is not a market loss. It is the same money, handed back voluntarily, in a handful of frightened moments.

Why this is the good news

Inflation and fees are largely outside your control. The behaviour gap is almost entirely inside it — which makes it the most fixable cost you will ever face. You do not close it by being smarter or watching more closely. You close it by removing the moments where emotion gets a vote: automate the buying, decide the rules in advance, and then leave the plan alone.

That is the entire argument of the Real Life Scenarios series — from why willpower fails to the three-check routine professionals run before every trade. The disciplined line in the chart above is not a reward for brilliance. It is the reward for being deliberately, profitably boring.

What this deliberately ignores

• Timing of the damage. Real behaviour gaps are episodic, not smooth — concentrated in a few crashes. The reaction rule here is a simplified stand-in, for clarity, not precision.
• Tax and inflation. Both further reduce what the final figures are worth to you. Use the inflation calculator alongside this one.
• That some gaps are rational. Selling for a genuine life reason is not a behaviour gap. The gap is the cost of selling for an emotional one.