The Split Deal

How it works

Ada and Bo are held in separate rooms and each offered the same deal: testify and your own sentence is wiped. Silent together they serve 1 year each; both testifying costs 8 each; one testifying while the other stays silent frees the talker and costs the silent one 10. The story shows the full payoff matrix, then asks what each one does. You type a guess, lock it in, and step through the answer and breakdown.

How scoring works

There is no grading - your typed guess is shown back beside the real answer. The story teasers share a single lifetime unlock for free users, so one reveal locks the group.

The key insight

Fix the other player's choice and compare your own options against it. Whatever Bo does, Ada is better off testifying - a dominant strategy - and the game is symmetric, so both testify and serve 8 years even though mutual silence costs 1 each. The equilibrium is stable precisely because neither can improve alone, and it is still worse for both than the outcome they rejected.

The full step-by-step solution lives at /brain-teasers/prisoners-dilemma/solution.

A worked example

Suppose Bo stays silent: Ada gets 1 year silent, 0 testifying - testify wins. Suppose Bo testifies: Ada gets 10 silent, 8 testifying - testify wins again. One option won both cases, so Ada never needs to guess Bo's move, and Bo runs the identical reasoning.

Common mistakes

• Answering 'both stay silent because it is best overall' - jointly best and individually stable are different properties.

• Trying to reason about both players' moves at once instead of fixing one and comparing.

• Calling mutual silence an equilibrium - either player gains by defecting from it.

Why interviews test this

Interviewers use this to check you can find a dominant strategy case by case and attach the terms 'dominant strategy' and 'Nash equilibrium' to the right cells. The written solution at /brain-teasers/prisoners-dilemma/solution covers what breaks the trap.