Most people who try day trading lose money. The reasons are predictable, structural, and largely self-inflicted.

The logical errors that do the most damage #

Everyone talks about sunk cost bias: you keep feeding a position because you've already fed it too much. You know the trade is dead, but you keep paying tuition.

That's not the only bias working against you.

Many traders convince themselves they can predict short-term moves with enough indicators, enough screens, or enough "edge." They cannot.

Another classic: you remember your winners like movie highlights and memory-hole your losers. That gives you fake confidence and terrible sizing.

And some people eventually trade compulsively. At that point, logic leaves the room and position size becomes mood regulation.

Addressing the unconscious biases #

Your brain is built for speed, not market truth. It uses shortcuts from past experiences, beliefs, and ego protection. Useful in daily life. Dangerous in markets.

• Sunk cost bias: You keep investing because you've already invested, not because the thesis still works.
• Survivorship bias: You study visible winners and ignore the graveyard.
• Selection bias: You compare yourself to cherry-picked outliers and call it a plan.
• Compulsive trading: You chase emotional relief, not expected value.

Modern portfolio theory (MPT) #

Modern Portfolio Theory is not sexy, and that's exactly why it works. The core idea is simple: portfolio construction matters more than hero trades.

It forces you to think in probabilities, diversification, and risk-adjusted returns instead of vibes.

What MPT gives you:

1. A framework for balancing risk and return.
2. A discipline for position sizing and diversification.
3. A way to evaluate results without lying to yourself.
4. A defense against over-concentration and narrative addiction.

Key Indicators of MPT #

1. The efficient frontier is the set of portfolios with the highest expected return for a given risk level.
2. The capital asset pricing model (CAPM) is a model that describes the relationship between expected return and risk for a security or portfolio.
3. The Sharpe ratio tells you how much return you got per unit of volatility.

You can keep pretending you're one indicator away from mastery, or you can build a system that survives your own biases.