This is the reverse of a DramaticIdentity. With a DramaticIdentity, multiple people might post under the same moniker. With a Sock Puppet, the same person posts under multiple identities. Some of the alternate identities may be automated "bots", but more often they're under manual control.
Use of a SockPuppet is typically frowned on. The social convention against sock puppets arises from the RealWorld. We do not expect to discover that the guy who runs the coffee shop is also our neighbour, but with a fake mustache and beard. In most communities there is at least a social expectation, and sometimes a site policy, that each person will use only a single identity. Thus, use of sock puppets is a DeceptivePractice.
A PenName (or anonymous posting) is about discretionary exposure of personally identifying information, and doesn't exclude the possibility of participating with a measure of integrity since discretion and deception are not at all the same thing. It is fair to expect that we all exercise a certain measure of discretion, but it is hoped that we do not engage in intentional deception. A SockPuppet, on the other hand, is dishonestly represented as being a unique individual, distinct from the existing identities already in use in the community.
However, a sock puppet can be open about the fact that it is a sock puppet. For example, a forum host might choose to participate under two names, one, say "Sys-op", for when they are giving official pronouncements as host, and one, say "Tall Boy", for when they are giving off-the-cuff personal opinions as an ordinary participant. There need be no intent to deceive here. Both names would be PenNames, but everyone would know they referred to the same real-world person.
By using several Sock Puppets all posting in favour of an idea, a dishonest person can give the impression that the idea has more grass-roots support than it really has. See also StuffingTheBallotBox, which is a similar problem in a more formal, voting context. Sock puppets make it harder to enforce "one man, one vote."
Sock puppet identities are disruptive in conversation because they are often used so that a person can ask questions of himself and appear to be carrying on a thread between two individuals.
Sock puppets break SerialIdentity and thus IteratedPrisonersDilemma. They can defeat KillFiles (c.f. CommunityWiki:IgnorePeople) and reduce the effectiveness of other forms of rating/moderation that has different per-user behaviour (e.g., SlashDot "karma"). They also work against SoftSecurity attempts to DissuadeInteraction, EnforceResponsibility, and AssumeGoodFaith. In a similar light, sock puppets are the most frequent means to circumvent attempts to apply an involuntary CommunityExile.
Use of a SockPuppet may also be a means for a member to avoid linking too visibly a RealName activity from a PenName activity on the net. It is not necessarily meant to be disruptive in the community itself. Thus someone may choose to have a RealName identity and a PenName identity in the same community.
Some communities, such as TheFreshPress and CafeUtne, have formal policies against multiple identities by non-administrative participants. If you switch identity, then the password is changed on your old identity so you can't use it. How well this LegalSolution works in practice is debatable
Any medium which is effectively "world-writeable" is vulnerable to the SockPuppet phenomenon, whether wiki or PicoSpan inspired without identity verification. By contrast, TheWELL requires a monthly payment for each user ID, providing an EconomicSolution to sock puppetry. Thus it is a minimally GatedCommunity.
Communities that make it relatively easy to switch UserIDs?, or that do not use them at all, provide relatively lower barriers to the use of Sock Puppets compared to sites that have more involved login/logout procedures. Sites that prominently identify the current logged-on user make it easier to conduct sock puppet conversations without slipping up.
eBay tacitly encourages use of multiple identities. Many books about eBay encourage sellers to use multiple identities, and virtually all discussion participants use a separate user ID than they use for selling.
I generally think it's a good thing when a forum host posts under 2 different identities, one for official announcements and one for idle chit-chat.
And I am very sympathetic to people who switch to a different pen-name every couple of years to avoid drawing unwanted attention from those who refuse to ForgiveAndForget.
Yes, "use-once throw-away identities" breaks one solution to some problems. And I agree that anything that breaks serial identity also breaks the IteratedPrisonersDilemma.
More and more I am seeing that, when a problem occurs, the first solution that comes to my mind is rarely the best solution — and occasionally the "solution" turns out to be a counter-productive Wiki:CureWorseThanTheDisease.
When "something keeps getting messed up", usually the first solution that comes to mind is "lock out all the people who mess it up". I am delighted to find that there is another solution to this problem, one implemented by (most) wiki: "make it easier to undo the mess-ups than it is to mess things up". This solution is (potentially) superior, because (unlike the first solution) it works even though all humans eventually mess up. -- DavidCary