Building Web Reputation Systems: The Blog

Because an underlying karma score is a number, product managers often misunderstand the interaction between numerical values and online identity. The thinking goes something like this:

• In our application context, the users' value will be represented by a single karma, which is a numerical value.
• There are good, trustworthy users and bad, untrustworthy users, and everyone would like to know which is which, so we will display their karma.
• We should represent good actions as positive numbers and bad actions as negative, and we'll add them up to make karma.
• Good users will have high positive scores (and other users will interact with them), and bad users will have low negative scores (and other users will avoid them).

This thinking—though seemingly intuitive—is impoverished, and is wrong in at least two important ways.

• There can be no negative public karma-at least for establishing the trustworthiness of active users. A bad enough public score will simply lead to that user's abandoning the account and starting a new one, a process we call karma bankruptcy. This setup defeats the primary goal of karma-to publicly identify bad actors. Assuming that a karma starts at zero for a brand-new user that an application has no information about, it can never go below zero, since karma bankruptcy resets it. Just look at the record of eBay sellers with more than three red stars-you'll see that most haven't sold anything in months or years, either because the sellers quit or they're now doing business under different account names.
• It's not a good idea to combine positive and negative inputs in a single public karma score. Say you encounter a user with 75 karma points and another with 69 karma points. Who is more trustworthy? You can't tell: maybe the first user used to have hundreds of good points but recently accumulated a lot of negative ones, while the second user has never received a negative point at all. If you must have public negative reputation, handle it as a separate score (as in the eBay seller feedback pattern).

Even eBay, with the most well-known example of public negative karma, doesn't represent how untrustworthy an actual seller might be-it only gives buyers reasons to take specific actions to protect themselves. In general, avoid negative public karma. If you really want to know who the bad guys are, keep the score separate and restrict it to internal use by moderation staff.

Virtual Mafia Shakedown: Negative Public Karma

The Sims Online was a multiplayer version of the popular Sims games by Electronic Arts and Maxis in which the user controlled an animated character in a virtual world with houses, furniture, games, virtual currency (called Simoleans), rental property, and social activities. You could call it playing dollhouse online.

One of the features that supported user socialization in the game was the ability to declare that another user was a trusted friend. The feature involved a graphical display that showed the faces of users who had declared you trustworthy outlined in green, attached in a hub-and-spoke pattern to your face in the center.

People checked each other's hubs for help in deciding whether to take certain in-game actions, such as becoming roommates in a house. Decisions like these are costly for a new user – the ramifications of the decision stick with a newbie for a long time, and backing outof a bad decision is not an easy thing to do. The hub was a useful decision-making device for these purposes.

That feature was fine as far as it went, but unlike other social networks, The Sims Online allowed users to declare other users un trustworthy too. The face of an untrustworthy user appeared circled in bright red among all the trustworthy faces in a user's hub.

It didn't take long for a group calling itself the Sims Mafia to figure out how to use this mechanic to shake down new users when they arrived in the game. The dialog would go something like this:

"Hi! I see from your hub that you're new to the area. Give me all your Simoleans or my friends and I will make it impossible to rent a house.”

"What are you talking about?"

"I'm a member of the Sims Mafia, and we will all mark you as untrustworthy, turning your hub solid red (with no more room for green), and no one will play with you. You have five minutes to comply. If you think I'm kidding, look at your hub-three of us have already marked you red. Don't worry, we'll turn it green when you pay…"

If you think this is a fun game, think again-a typical response to this shakedown was for the user to decide that the game wasn't worth $10 a month. Playing dollhouse doesn't usually involve gangsters.

Avoid public negative reputation. Really.