(also known as: false dichotomy*, the either-or fallacy, either-or reasoning, fallacy of false choice, fallacy of false alternatives, black-and-white thinking, the fallacy of exhaustive hypotheses, bifurcation, excluded middle, no middle ground, polarization)
Description: When only two choices are presented yet more exist, or a spectrum of possible choices exists between two extremes. False dilemmas are usually characterized by “either this or that” language, but can also be characterized by omissions of choices. Another variety is the false trilemma, which is when three choices are presented when more exist.
Either X or Y is true.
Either X, Y, or Z is true.
Example (two choices):
You are either with God, or against him.
Explanation: As Obi Wan Kenobi so eloquently puts it in Star Wars episode III, “Only a Sith deals in absolutes!” There are also those who simply don’t believe there is a God to be either with or against.
I thought you were a good person, but you weren’t at church today.
Explanation: The assumption here is that bad people don’t go to church. Of course, good people exist who don’t go to church, and good church-going people could have had a really good reason not to be in church -- like a hangover from the swingers' gathering the night before.
Exception: There may be cases when the number of options really is limited. For example, if an ice cream man just has chocolate and vanilla left, it would be a waste of time insisting he has mint chocolate chip.
It is also not a fallacy if other options exist, but you are not offering other options as a possibility. For example:
Mom: Billy, it’s time for bed.
Billy: Can I stay up and watch a movie?
Mom: You can either go to bed or stay up for another 30 minutes and read.
Billy: That is a false dilemma!
Mom: No, it’s not. Here, read Bo’s book and you will see why.
Billy: This is freaky, our exact conversation is used as an example in this book!
Tip: Be conscious of how many times you are presented with false dilemmas, and how many times you present yourself with false dilemmas.
* Staying true to the definitions, the false dilemma is different from the false dichotomy in that a dilemma implies two equally unattractive options whereas a dichotomy generally comprises two opposites. This is a fine point, however, and is generally ignored in common usage.