St. Petersburg (style) Paradox

A biased coin game: heads doubles your capital, tails halves it. Starting capital: $100. The more rounds you play, the higher your expected winnings—but also the more certain your ruin.

1/3 ≈ 0.333 1/2 = 0.5
100 1000

Final Capital

$0.00

0% change

Expected Value

$0

Typical Outcomes (±1σ)

$0 — $0

Heads / Tails

0 / 0

Actual Heads %

0%

Capital Over Time

Note: Y-axis uses logarithmic scale to show exponential growth/decay

📊 The Paradox Explained

Expected growth per round: +0% — The expectation value suggests profits!

Typical growth per round:0% — But in the long run, you typically lose many more rounds than you win.

The rare huge wins inflate the expected outcome, but the player typically goes broke.

The probability of winning an individual round (doubling the capital) is p < 1/2.

Expected capital after n rounds: E[Xn] = X0 · ((1 + 3p)/2)n → ∞

Typical capital after n rounds: Xn ≈ X0 · 2n(2p−1) → 0