Can peer-to-peer (P2P) reputation systems work?
Some of the successful and well known reputation systems on the web are controlled and managed by a central entity. An example that I have used in the past is the eBay feedback system. eBay enables buyers and sellers to rate other buyers and sellers, and presents that reputation information to everyone. Another example is the industry for digital certificates in which there are a few large, trusted "authorities" who vouch for the reputation of the certificate holder. In both of these examples, the reputation system is controlled by a large corporation - giving that company a lot of power, competitive leverage, etc.
Since power in the hands of large corporation can easily corrupt, the idea of a peer-to-peer reputation system - one that is not controlled by a single entity - is very appealing to many (including myself). But can such a system work? And if so, how?
First, I think we need to look at the main reasons why centralized reputation systems are successful. One of the most important reasons is trust. These systems work because people trust the reputation information presented by them. When look at the eBay reputation of someone, I have a high degree of confidence that the person's feedback score is accurate - I don't stop to wonder if that person has hacked the eBay system and changed the score. The eBay system works a s a reputation system because people trust eBay.
But in a P2P approach, there is no large, recognizeable organization. Reputation information would be hosted in many different places, perhaps on the web sites or computers of individuals whom you do not know. So how do you know that you can trust these people? In order to trust them, you need to learn about their reputation.....here the problem becomes circular in nature. Can can we be confident that reputation information information in a P2P system is not forged or manipulated? One solution that comes to mind quiclky is that the reputation information could be stored in more than one place - in which case an anomoly could indicate attempts to forge data. But what happens when reputations legitmately change - the change must propagate to other nodes - how do you know that it is a real change and not a forged change? How can we trust reputation information in a peer to peer system?
For the answer, I think we need to look at the most common reputation system of them all, one that has been around for thousands of years: word of mouth. For moment, let us forget about the web, TV, radio, newspapers - without these sources of information, how do we learn about the reputation of others? Personal interaction is the first way we can do this. If we buy products from a vendor at a (social) market, based on the quality of the products and the vendor's claims, we develop and opinion about the reputation of that person. Another way is by talking to others, sharing our opinions about the reputations of others. If I believe that the market vendor has a good reputation, and John believes the same, it reinforces my opinion of the vendor's reputation. But what if Mary tells a story about the vendor in which she was cheated? That might decrease my opinion of the vendor. This also works for people we have never met. Before I walk into a new shop, I might ask a few people about their opinions and experiences with the shop owner. Some of the people I ask may not have been in the shop either, but may relate stories that others have told them. This way I gather reputation information for someone that I never met - which is exactly what we need to do on the web! but what happens if I get 10 conflicting opinions about the shop owner? How do I use these to form my own opinion? The answer again is trust. Some opinions I will weight heavier than others, depending on how much I trust the person giving the opinion - depending on their reputation. I trust the opinion of my best friend more than that of a casual aquaintance. This raises an important difference between a centralized reputation system and a word-of-mouth system: with a word-of-mouth reputation system, reputation is relative, not absolute. In the real world, a person's reputation is seen throught the eye of the beholder. Can we harness this word of mouth system on the Internet?
The word-of-mouth system, as described above relies on a social network to gather and disseminate reputation information. Online social networks can be used in the same way, whether they are centralized systems or peer-to-peer. Now we can revisit the question: how can we trust reputation information in a peer to peer system? The answer is that you trust the people that you know best, just like in the word-of-mouth system. Online social networks already track the friend-of-a-friend and six degrees of seperation, and P2P applications can do the same. Members just need to input the reputation information for the people that they know directly, and reputation information could aggregated and persoanlized based on each person's trust level and degrees of separation. This way you don't need a big corporation to "trust", you just need to trust your friends. Perhaps some of the online social networks are already moving in this direction, I am not sure. But it seems to me that the answer is "Yes", P2P reputations systems can work.
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