The Gish Gallop is a debate tactic named after creationist Duane Gish, who has a reputation for performing this technique in open debates. The Gish Gallop involves throwing so many lies, half-truths, misconceptions, and straw man arguments at one's opponent that he cannot possibly address all of them. The Gish Gallop works very well in open debates, but fails spectacularly in interviews, where the interviewer is able to maintain control of the conversation.
The reason the Gish Gallop works so well is because it is very quick and easy to make a statement, and very difficult to refute it, regardless of how inaccurate the statement is. A creationist can say, for example, that we have never found evidence of life spontaneously generating from non-life in jars of peanut butter. The reasons why we don't, why it's not important, and why it doesn't even fall in the realm of evolutionary theory are lengthy and complex to explain. The statement itself is easy for a non-technical audience to digest, and breaking the refutation down into lay-friendly explanations can make it even more difficult.
The Gish Gallop takes this imbalance in the difficulty in throwing out misinformation versus refuting it and magnifies it by throwing out as much as possible. What takes a minute or two to say could take hours to comprehensively refute, if the opponent can even remember all the problematic statements that were even said. Picking only the most damaging or easily refuted points still leaves piles of problematic statements the audience may believe.
Because of tactics like the Gish Gallop, it can be very difficult for an unprepared opponent to engage in debate with people who are willing to use them. Conspiracy theorists, creationists, promoters of alternative medicine, and pseudoscientists are all famous for using such tactics in open debate. A typical debate format consists of a set time for each side to make their points, followed by a set time in which they may challenge each other's points, taking turns speaking and not allowed to interrupt each other. These time limits work very much in favor of anyone willing to use the Gish Gallop.
The term Gish Gallop was coined by Eugenie Scott, executive director of the National Center for Science Education. Eugenie Scott is a leading challenger of evolution denial and has written extensively on the subject of avoiding illegitimate debate tactics. The best defense against the Gish Gallop is to be able to interrupt one's opponent and challenge the bad statements as they arise, which is why interview formats are particularly damaging to the tactic.
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