Abstract. In many Web-based applications, there are incentives for a user to sign up for more than one account, under false names. By doing so, the user can send spam e-mail from an account (which will eventually cause the account to be shut down); distort online ratings by rating multiple times (in particular, she can inflate her own reputation ratings); indefinitely continue using a product with a free trial period; place shill bids on items that she is selling on an auction site; engage in falsename bidding in combinatorial auctions; etc. All of these behaviors are highly undesirable from the perspective of system performance. While CAPTCHAs can prevent a bot from automatically signing up for many accounts, they do not prevent a human from signing up for multiple accounts. It may appear that the only way to prevent the latter is to require the user to provide information that identifies her in the real world (such as a credit card or telephone number), but users are reluctant to give out such information
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