Guilt by association is a form of association fallacy in which someone makes conclusions with the use of assumptions based on the traits of people, ideas, and things which are actually unrelated. For example, one might say “humans walk on two legs, bears walk on two legs, therefore bears are human.” This extreme example of guilt by association demonstrates how this particular cognitive fallacy works, but it is usually much more subtle than this. The other type of association fallacy is honor by association, in which someone makes assumptions about positive traits, like “any friend of Jane's is a friend of mine.”
Like other cognitive biases, guilt by association is designed to help people function in the world by allowing them to make quick judgments. Sometimes, this can be a good thing, as cognitive biases can allow people to interpret information quickly and sometimes surprisingly accurately. In other instances, cognitive biases get in the way of actually evaluating information, and they can lead people into logical traps.
People often use this fallacy in rhetoric, in appeals to the emotions of readers and listeners. For example, a politician might say “my opponent has been seen in the company of a radical activist, therefore my opponent is also a radical activist.” Plays on emotion often feature symbols with which everyone is familiar, such as Hitler as a symbol of evil in an argument like “Hitler supported euthanasia, so euthanasia must be evil.” People can also use honor by association in rhetoric.
In the heat of discussion, sometimes it is hard to catch guilt by association, especially when it is well-deployed. People often use it in an attempt to trick people into rejecting arguments, as in “someone you dislike supports this argument, so the argument must be bad,” with honor by association being used to lure people into believing an argument, as in “the political organization you support stands behind this ballot initiative, so the initiative must be sound.” Often, guilt by association is used as a form of red herring, to distract someone from the actual meat of the argument.
This concept pops up in a number of circumstances. In legal cases, for example, lawyers often try to manipulate the jury with guilt or honor by association, just as politicians coax citizens into supporting or opposing causes with association fallacies. Many people find themselves using association fallacies on a regular basis, to do everything from convincing a child to eat vegetables ("your favorite superhero eats carrots, so they must be cool") to arguing with a friend about politics ("did you know that your candidate accepted funding from such-and-such organization?").