Argument from adverse consequences
"If X were true, then it would imply Y. Y is bad, therefore X is not true."
This argument is invalid: just because something is bad does not mean that it's not true.
The form of this argument resembles, but is different from reductio ad absurdum, which is of the form "If X were true, then it would imply Y. But Y contradicts X. Therefore X is not true." Reductio ad absurdum is a valid argument; the argument from adverse consequences, on the other hand, is a fallacy because it does not argue that the consequence Y cannot happen or contradicts X, but merely that it is undesirable.
This should also not be confused with planning. It is quite valid to say, "If the government raised taxes, the economy would suffer. This would be a bad thing, therefore the government should not raise taxes." The argument from adverse consequences is about whether some statement is true, not whether some action should be taken.
- "If my father had committed the crime of which he's accused, our family would be disgraced. Therefore, my father is innocent."
- "If evolution is true, then life is meaningless. Therefore evolution is false."