Risk of ruin is the probability that a string of losses wipes you out before your edge can play out. The unsettling truth it reveals: you can have a genuinely profitable strategy and still go broke if you size too large — because losing streaks are inevitable, and a big enough one ends the game permanently.
Why streaks are guaranteed
Even a 60%-win strategy will, over enough trades, hit long losing streaks — five, eight, ten losses in a row are statistically certain to happen eventually. If each loss costs 1% of your account, a ten-loss streak is a survivable ~10% dent. If each costs 10%, the same streak is a catastrophic ~65% drawdown you may never recover from. Same strategy, same streak — only the sizing differs, and that determines whether you survive.
The non-negotiable rule
Keep per-trade risk small enough that a realistic worst-case losing streak cannot ruin you. This is why 1–2% per trade is the standard: it makes ruin from normal variance effectively impossible, buying you the many trades needed for a positive expectancy to assert itself. Over-leverage does the opposite — it shrinks the streak required to end you, so variance alone eventually finds you out. The market does not even need to be 'wrong'; it just needs a normal bad run.
The deeper lesson
Survival is the precondition for everything else. An edge only pays if you are still trading when it shows up, so the trader's first job is not to maximise returns but to make ruin impossible — then let a positive expectancy compound over time. This is the quantitative backbone of why this whole project prizes 'survive a clean season, zero loss-of-funds' over chasing a green curve: you cannot compound from zero.