Joe Carter recently pointed out a fallacy called the Argumentum ad consequentiam, explaining,
The author points to the disagreeable consequences of holding a particular belief in order to show that this belief is false. (Ex. You can't agree that evolution is true, because if it were, then we would be no better than monkeys and apes.)In the comments, Jon Rowe wonders what separates this from a reductio ad absurdum. The difference is whether the consequences are logical, and thus create an inconsistency (as in a reductio) or a red herring unrelated to the proposition's truth value.
For instance, a child's belief in Santa Claus may have good consequences in making the child happy and well-behaved, but these facts have nothing to do with whether there really is a Santa Claus.Kissing cousin to the appeal to consequences is the argument to false motives: "You believe x only because it benefits you or permits you to do something objectionable." This is also known as a circumstantial ad hominem.
Both fallacies crop up inordinately in debates about evolution, personhood, or atheism (or, often, all three at once). The latter is especially pernicious. Consider Carter against a functionalist view of personhood:
The blatant attempts at rationalizing clearly immoral behavior is why Frank Beckwith and other scholars have been able to demolish the “functionalism” argument, that defends the killing of "non-person" humans.... If you want to kill certain groups of human beings, you can find a sufficient rationalization.Or, as he's echoed elsewhere, "Also, isn't it odd that the only times people make distinctions about 'human being' and 'human person' is when they want to treat members of the human species as sub-human?" Strangely, this comes from the same writer who immediately before that (in the same comment thread!) made the same distinction, and previously was open to reconsidering whether life begins at conception, or, alternately, at implantation--presumably without wicked motives.
Watch out for the argumentum ad consequentiam and the circumstantial ad hominem. Their lure is powerful, even to those who are wise to their ways.