This is the fallacy of asserting that there are only two sides to an issue or two answers to a problem. As usual, this can be used unintentionally by someone or intentionally in an attempt to get someone to side with the arguer. This is usually set up in such a way (especially when intentional) that either you “agree” with the arguer, or you “agree” with some absurd position. For example [insert creationist sneer]:
Either you agree with evolutionists and think that all animals and plants just popped into existence out of random chance or you agree with the biblical account of creation which says god created everything.
I chose an obvious example to illustrate the point, but it can often be much harder to spot. In this example, there are two absurd positions and the arguer, a creationist in this case, assumes that his/her position is less absurd and you’ll agree with him/her if you’re not an idiot. This is easy enough to counter, even if you don’t want to debate about evolution; simply point out that there are more than those two possibilities with regards to that issue.
The harder dichotomy to spot is one that you set up yourself. It’s hard to train your brain to think about multiple potential solutions to every problem you consider. For example, if you are one of those people who think that religion isn’t true, but is good because it provides a moral framework, what does this mean? It means that if there is something else that provides a moral framework better than religion, you need to toss religion out. When people employ this type of false dichotomy, they usually do it in the context of a post hoc rationalization. The above example goes something like this:
Ok, so my religion isn’t true. And fine, the several examples of largely atheistic countries flourishing show that atheism won’t destroy society. But religion is needed for a moral framework!
This is an example of what is called rationalization. It’s when you look for any reason at all, even if it’s not a good reason, to continue believing something that you want to believe. The reason I’m putting it on a post about the false dichotomy is that the implicit reasoning behind this type of rationalization is an implicit dichotomy. “Either religion provides the best moral framework, or nothingness does.” For more about this type of dichotomy (and spelled out much better) check out this post on LessWrong called The Third Alternative.