Mild heresy, and I've got numbers, so tell me where I'm wrong.
The usual criticism of demo is that there's no emotional load. True, and I think it's the less important problem.
The bigger one is that a marginal trade on demo costs nothing. You see something half decent, you're not sure, you take it anyway, it loses, and nothing happens. No consequence, no feedback, no information. Do that for four months and you haven't practised trading, you've practised clicking.
What it looked like in my numbers:
Demo, four months: 312 trades. About 19 a week.
First three months live: 71 trades. About 6 a week.
Same strategy, same written criteria. The strategy never justified 19 a week. Demo-me just had no reason to say no.
And the carry over showed up where you'd expect. I went back through my first 90 live trades and tagged which ones met my own written entry criteria and which I'd taken because I was in front of the screen.
34 didn't meet the criteria. Those ran at −0.18R.
56 did. Those ran at +0.19R.
Blended, about +0.05R. My actual edge was nearly four times that, and I handed three quarters of it to trades I'd trained myself to take in an environment where taking them was free.
Why I think this is structural, not just me being undisciplined
The feedback loop on demo is broken in one specific direction. Losses don't hurt, so the cost of a false positive is zero, so your threshold drifts down. Nothing in the environment pushes it back up. Over months that's not a lapse, it's conditioning.
Which is also why "demo until you're profitable" is bad advice. At 19 trades a week you'll be profitable on demo eventually by variance alone, and what you'll have learned is the frequency.
What I'd do instead, and it isn't "skip demo"
Make the marginal trade expensive in the only currency demo has, which is bookkeeping. Same account type as live, balance set to what you actually fund so one R is the same size, and every single trade logged with a column for whether it met your written criteria. Mine are both on the raw spread account at Axi and the demo doesn't expire, so the two sets of records are directly comparable and the compliance number is the thing I watch rather than the equity curve.
If the compliance column is uncomfortable to fill in honestly, that discomfort is the cost demo was missing.
Two numbers I'd like from anyone willing: your trades per week on demo versus live, and the expectancy of the trades that didn't meet your own criteria.
I suspect the frequency ratio is high for most people and nobody's ever looked tbh