Deductive reasoning is the process of reaching a conclusion that is guaranteed by the statements you start from. If the premises are true and the argument is built correctly, the conclusion cannot be false. On a reasoning test, that means you are given some statements and asked what must follow from them, not what is probably true or what you happen to believe.
Premises and conclusions
Every deductive argument has two kinds of parts. The premises are the statements you are asked to accept. The conclusion is the claim that is supposed to follow from them. In the standard textbook case, "All mammals are warm-blooded" and "Whales are mammals" are the premises, and "Whales are warm-blooded" is the conclusion.
The word that does the work is "follows". A conclusion follows when there is no way to make every premise true and the conclusion false at the same time. That is a stricter standard than "sounds reasonable" or "is probably right". In a deductive question only what the statements force counts. Your general knowledge, your hunches and the plausibility of an answer are all irrelevant.
Validity is about structure, truth is about facts
Two words are confused more often than any others in this topic. An argument is valid when its conclusion follows from its premises. A statement is true when it matches the world. Validity belongs to the shape of the argument, and truth belongs to individual claims.
The two can come apart in both directions. "All fish live in salt water. Trout are fish. So trout live in salt water" is a valid argument with a false first premise, and its conclusion is false in reality. "Some doctors are parents. Some parents are tired. So some doctors are tired" has premises and a conclusion that all sound true, yet the argument is invalid, because nothing ties the tired parents to the doctors.
An argument that is valid and also built on true premises is called sound, and only a sound argument gives you a conclusion you can rely on in the real world. Reasoning tests usually ask about validity alone and expect you to treat the premises as true, even when they are strange. The skill being measured is following the logic, not checking the facts.
The main forms you will meet
Most deductive questions are built from a handful of forms, and each has its own typical traps.
- All, no and some statements. "All invoices need a signature. This document is an invoice. So it needs a signature." These are covered in the guide to syllogisms, validity and truth.
- If-then rules. "If the door is locked, the alarm is on. The alarm is off. So the door is not locked." The two valid patterns and the two classic traps are in the guide to if-then reasoning.
- Ordering. "Kai is older than Lou, and Lou is older than Mei, so Kai is older than Mei." Ranking puzzles chain these comparisons together.
- Either-or. "The file is in the red folder or the blue one. It is not in the red one. So it is in the blue one."
What deduction does and does not do
A valid deduction adds no new information. The conclusion only makes explicit something the premises already contained. That is exactly why it is reliable: nothing has been smuggled in.
It is also why deduction is a poor tool for discovering new facts about the world, which is the job of its counterpart, induction. To see how the two differ and how to tell them apart in a question, read deductive and inductive reasoning explained.
People have studied valid argument forms for well over two thousand years, so none of the forms above is new. What is new for most test-takers is having to apply them quickly, with unfamiliar content, and without letting what they already know leak into the answer.
How the mind does it, and where it slips
Psychologists do not agree on how people carry out deduction. One tradition, associated with Lance Rips, proposes that we apply rule-like steps, a little like writing a proof in our heads. Another, developed by Philip Johnson-Laird, proposes that we imagine the possibilities the premises allow and then check whether the conclusion holds in all of them. Both camps agree on the practical point: errors are not random.
Two sources of error are well documented. The first is the pull of belief: a conclusion that sounds true is easier to accept than one that sounds false, even when neither follows.
The second is speed. Jonathan Evans and others describe a quick intuitive response that usually arrives first, and a slower, more deliberate check that has to be actively switched on. Deductive questions are designed so that the first answer that comes to mind is sometimes the wrong one.
One habit that works on almost every question
Ask yourself: can I make every premise true and the conclusion false at the same time? If you can build even one such case, the conclusion does not follow. Take "All poodles are dogs, so all dogs are poodles". Poodles and other breeds make the premise true and the conclusion false, so the argument fails.
Notice that this habit never asks whether the conclusion is plausible. It asks only whether the premises force it. The same habit is the backbone of the step-by-step method in how to solve deductive reasoning questions, and it is the quickest way to catch the slips listed in common deductive reasoning mistakes.
Where to practise
To feel the difference between "follows" and "sounds right" under test conditions, try the JobCannon Deductive Reasoning Test. It is built around "follows / does not follow" conclusions drawn from a set of statements, using syllogisms, if-then rules and ranking puzzles.
JobCannon also has an older logical reasoning test, a broader mix that adds analytical puzzles. The questions are written in-house, so treat your result as practice feedback rather than a formal measure of ability.