Often on the internet there is a trade-off between anonymity and accountability. For privacy and security reasons, a lot of users want to stay anonymous. But this contradicts services like Wikipedia who want to make sure that their content is of high quality and so need a certain kind of accountability of the users.
Also the Tor-network has this problem, that due to the anonymity it provides, some users can abuse the network to attack websites by impersonating a multitude of users, while in reality they are only one entity. So a lot of websites present a Captcha when visited through the Tor-network, to prevent some kind of spamming attacks.
Captchas are not convenient: sometimes it is even difficult for humans to solve them, computers get better and better at solving them automatically, and they still don't protect from somebody opening multiple accounts by solving multiple Captchas.
Proof of Personhood is one solution to this problem: if we can create one token for one person, then we can use that token to prove to the service that we are a human being. One readily such available token are passports or other id-cards. But these cards are expensive and don't allow for anonymity! In Pseudonym Parties, Bryan Ford proposes in-person meetings, where each person gets exactly one token, with which he can prove that he was at a given time at a given place, and thus prove he's a human being.
Work done by Maria Fernanda Borge Chavez at the DEDIS-lab at EPFL worked out how we can use cryptography to create a token that can prove a person knows a secret, without giving away which person of a group it is. Furthermore, if we have different services like Wikipedia, Tor, or even voting systems, it is possible that each service gets its own view of the group and that they cannot collude to learn more about connections between the user.