The Fallacy Of Moving The Goalpost

This post first appeared on Lernabit.com. Lernabit helps you remember everything you learn about any topic.

Moving the goalposts is a fallacy in which the two parties in a debate agree on evidence that would refute a claim, but then, when such evidence is presented, the “losing” side insists that the given evidence is insufficient. At first this might not seem like a fallacy, because while it is obviously a cheap shot to use in a debate, the logical implications of it are not so obvious. But as we will see, it is, in fact, a logical fallacy.

An example of moving the goalpost can be found when discussing the topic of evolution, as in this example argument:

Bob: If evolution is real, then show me an example of evolution occurring right now.

Suzy: Sure. Just look at the rise of antibiotic resistant bacteria. As antibiotics are used, they apply selective pressure that weeds out those that are susceptible to it, allowing those that are resistant to grow out of control.

Bob: No, that doesn’t count. Show me an example that occurs over long periods of time.

In this debate, Bob is guilty of moving the goalpost. First, he suggests that an example of evolution occurring right now would make him change his position. But then, when such evidence is given, he changes his definition of evolution to exclude species changes that occur over short periods of time, effectively making it impossible to refute his claim. How can you possibly provide evidence of evolution occurring right now if his definition of evolution excludes rapid changes in species? You can’t. Bob either has to accept short term examples as sufficient evidence as previously agreed or allow the use of evidence to show long term changes, such as fossil records and carbon dating. By changing his requirements after the evidence has been presented, he creates a new claim that is impossible to refute.

As another example, consider homeopathy, which some people believe is a treatment for medical conditions, although there is no reliable evidence to support this claim. A discussion about the effectiveness of homeopathy might look something like this:

Jeff: Homeopathy works. Show me a study that proves otherwise.

Megan: Okay. Here is a double blind, placebo controlled study showing that it doesn’t work.

Jeff: Okay, but that’s one study. That doesn’t prove anything.

It is true that more evidence is always better, but after requesting a study showing the ineffectiveness of homeopathy, receiving said evidence, then changing his requirements, Jeff is moving the goalpost. This is especially true given that Megan provided a double blind, placebo controlled study, which is the gold standard of research.

In another lecture, I discussed a fallacy called the Texas sharpshooter fallacy, in which someone gathers evidence of their claim before clarifying the claim itself. Moving the goalpost is very similar, and is logically fallacious for the same reason. In short, it is a poor debate structure to wait for evidence before specifying the claim you want to prove. This is precisely what sets science apart from other methods of explaining the world.

Science is a process of developing a hypothesis, setting clear objectives that will either prove or disprove that hypothesis, and only then setting out to collect data. Progressing in this manner ensures that your preconceived biases don’t interfere with the research process. The Texas sharpshooter fallacy and moving the goalpost are fallacies because they reverse this process, thus reintroducing those biases back into the research.

An effective argument starts with premises backed by evidence, then uses those to draw a conclusion. And being an effective debater requires an open mind and a willingness to change your opinion in the face of new evidence rather than just moving the goalpost.