> **Interpretation:** ℛ₀ is the expected number of new deceptive "offspring" one deceiver produces in a fully-trusting market before being caught. If ℛ₀ < 1, honesty is evolutionarily stable. If ℛ₀ > 1, **deception necessarily invades** — this is the formal proof of Akerlof–Shiller's "phishing equilibrium."