Deductive reasoning can lead to incorrect conclusions through logical "fallacies."
DEFINITION: Logical "Fallacies" are weaknesses in either structure or content that render an argument either invalid or unsound (Giere, 2006).
Unfortunately, deductive reasoning does not always lead to true conclusions. Understanding the logical fallacies that cause some arguments to be invalid or unsound is important for science and critical thinking in general.
Understanding logical fallacies is important for science because scientists seek to avoid errors in reasoning. Scientific studies with faulty reasoning will most likely be rejected during peer-review and not published. Scientific papers with faulty reasoning that are published will become more and more inconsistent with other studies, and will eventually be discovered and dismissed. Therefore, logical fallacies do not benefit science or scientists. Scientists work to avoid fallacies, and structure arguments with the strongest reasoning possible.
Understanding logical fallacies is also important because many people try to use fallacious arguments to support arguments that are otherwise untenable. Unethical politicians or salespeople may use a number of strategies to convince people that flawed arguments are actually sound.
Two general categories of deductive fallacies are: 1) problems with the STRUCTURE of arguments that affect validity; and 2) problems with the FACTUAL basis of the premises that affect soundness.