Jason was a genuinely skilled trader. His system had a 58% win rate and a 1.8:1 reward-to-risk ratio — objectively profitable edges most traders would envy. Yet within fourteen months he'd blown a $50,000 account down to $6,200. His entries were solid. His exits were disciplined. His position sizing was catastrophic. He was running full-Kelly on every trade.

The Kelly Criterion tells you the mathematically optimal fraction of your capital to risk on any trade. The formula is: f* = (bp – q) / b, where b is the net odds (reward-to-risk), p is your win probability, and q is your loss probability (1 – p). For Jason's system: f* = (1.8 × 0.58 – 0.42) / 1.8 = 34.7%. That's the fraction Kelly says to risk. Per trade. It's mathematically optimal for long-run wealth — and practically brutal in real markets.

CONCEPTHalf-Kelly means risking exactly half your Kelly fraction — same edge, dramatically smoother equity curve.
WARNINGFull-Kelly drawdowns regularly exceed 30–40% even with a genuine edge. Most traders cannot hold through that psychologically.
KEY IDEAHalf-Kelly gives you 75% of full-Kelly's long-run growth rate at roughly half the variance. That trade-off is worth it.

Half-Kelly simply cuts that fraction in two: risk 17.3% of capital per trade in Jason's case. That still sounds large — because Kelly numbers often are. In practice, most professional traders apply Kelly to their overall system allocation, not individual trade sizing. They then layer a fixed fractional rule on top: never risk more than 1–2% of account equity on a single trade, regardless of what Kelly suggests.

$50k $100k $200k Trades (50) Full-Kelly Half-Kelly Equity growth ($)

Here's how the maths plays out on a $50,000 account with a 1% fixed-fractional rule. Risk per trade: $500. After a ten-trade losing streak — which happens to every profitable system — you've lost $5,000, sitting at $45,000. Recovery to breakeven requires an 11.1% gain, not 10%. Compare that to a 5% risk rule: ten losses wipe $25,000+, and you need a 100%+ recovery. Drawdown size determines survival. Smaller fractions protect the compounding base. The theoretical framework underpinning this is explored thoroughly in the literature on Kelly Criterion position sizing and connects directly to broader principles of money management in trading.

The Half-Kelly approach isn't about being timid — it's about staying solvent long enough for your edge to compound. Jason's system was always good enough. His bet sizing never was.

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