Logical fallacies of various kinds perennially show up in arguments in every debate. But there is one that seems to be the biggest one on the pro-21 or otherwise prohibitionist side. That one is the one we call the Reverse Middle Ground Fallacy, or Appeal to Extremes. It is basically a warmed-over form of the old slippery slope fallacy, in which the Middle Ground Fallacy or appeal to moderation is "debunked", and pretending that the opponent's entire argument is therefore "debunked" as well. It also functions as a straw man as well as appeal to logic.
For example:
- Claiming that there is no philosophically stable ground between communism and fascism, or alternatively between anarchy and totalitarianism, so we must pick one or the other
- Claiming there is no philosophically stable position that says that cannabis is OK but hard drugs are not, thus "confirming" the gateway theory
- Claiming that there is no philosophically stable ground between complete Luddism and complete technocracy or Transhumanism
- Claiming that reproductive rights or birth control of any kind inevitably leads to eugenics
- Claiming that tolerance and acceptance of homosexuality (or sexual freedom in general) inevitably leads to tolerance and acceptance of pedophilia and other horrible stuff, thus all LGBT+ folks are inherently "groomers"
- Claiming that gender equality is inherently impossible because one gender must utterly dominate and control the other, lest the latter run amuck and become too dangerous (note that this works both ways)
- Claiming that "you are either with us, or you are against us"
- Claiming that there is no philosophically stable ground between unfettered access to alcohol for all ages and complete and absolute alcohol prohibition for all ages (or for all people between some arbitrary age limit).
And yes, every single one of the above are examples of actual arguments that have been put forth by extremists of various stripes.
But simply reversing a fallacy, does NOT a valid argument make. Claiming that there is no philosophically stable ground between two extremes, just because one says so, is a sure path to Horseshoe Theory, where the extremes become far more alike than different (far left and far right, for example).
The 21 drinking age is not only a classic example of this fallacy, it actually undermines it's own position in its logical inconsistency and arbitrary selectivity compared to complete prohibition for all ages. Thus, the 21 drinking age, or any age limit higher than the legal age of majority, is the LEAST logically and philosophically stable position in regards to alcohol (or tobacco or cannabis or really anything else, for that matter) that could ever be conceived.
QED
UPDATE: Other notable fallacies (and questionable propaganda techniques) on the pro-21 side include:
- Ad baculum (appeal to the stick)
- Ad hoc (arbitrarily making it up as they go along)
- Ad hominem (of all types)
- Ad populum (appeal to the gallery)
- Alternative facts
- Ambiguity or equivocation
- Anecdotal "evidence"
- Appeal to authority (their favorite)
- Appeal to logic (the "fallacy fallacy")
- Appeal to novelty (or "progress")
- Appeal to tradition (yes, really!)
- Apples and oranges
- Assuming bad faith
- Citation mills
- Cherry-picking (selected instances)
- Contradictory arguments / kettle logic
- Cum hoc ergo propter hoc
- False dichotomy or false choice
- False "experts"
- Genetic fallacy
- Guilt by association (however tenuous, notably with Big Alcohol or fellow travelers)
- Hasty generalization
- Immune to evidence
- Junk science
- Moving the goalposts
- Mission creep
- Non sequitur
- Occam's Butterknife
- Oversimplification
- Poisoning the well
- Post hoc ergo propter hoc
- Projection
- Quote mining
- Red herring
- Slothful induction
- Straw man
- Suppressed evidence
- Using one bad policy to justify another
- Wishful thinking
