Four families (of Sybil-resistance)

The book's framing for the approaches the field is converging on, presented as composable layers rather than competing alternatives, each adding a property the previous lacked: (1) reputation — earned trust from accumulated history (PGP web-of-trust, eBay feedback, Advogato, EigenTrust/OpenRank); (2) identity — a persistent, costly anchor for that history, including biometric authentication (wallets, attestations, DIDs/VCs, passkeys, priced-entry forums, eIDAS wallets); (3) proof of personhood — uniqueness, one living human per enrollment (World ID, BrightID, Human Passport); (4) accountability — a traceable chain to an answerable principal (staking/slashing, Know Your Agent). None is a silver bullet; the engineering question is which combination fits which room, and the binding constraint is adoption, not invention.