Given the emphasis on Logical Reasoning on the LSAT, students often wonder if they are missing out by not taking formal (or deductive) logic in college. Granted, some exposure to deductive logic doesn't hurt: at their best, such courses will teach you the fundamental concepts of symbolic logic, help you understand the difference between valid and invalid arguments, and train you to use symbolic language to display the logical structure of complex arguments and statements. You will probably learn to analyze truth-functions ("and", "or", "not", "if...then"), as well as quantifiers ("all", "some"), which will ultimately help with conditional reasoning on the LSAT. Indeed, students who were exposed to formal logic before undertaking a test prep course with us seem to derive some benefit from it, judging from their performance in class and on their practice tests. However, think twice before you register for a Logic course at your local college this coming Fall. Here's why:
First, there is always the question of whether students who take formal logic and then ace the LSAT do so because they of their prior exposure to formal logic, or whether their have a natural predisposition toward logical thinking led them to excel at both. A classic causal dilemma, which is difficult to test.
Second, a college-level course requires a significant investment of time, which is arguably better spent doing something that is directly relevant to mastering the LSAT, such as reading the Bible Trilogy or taking an actual prep course designed specifically to get you ready by September (or December).
Third, the costs of taking a formal logic course sometimes outweigh the benefits. This is because the LSAT does not emphasize formal logic. On the contrary: test-makers have made a concerted effort (especially in recent years) to scale back the number of LR questions that do so, because such questions disproportionately benefit students who have been exposed to formal logic. Test-makers like to believe that the LSAT measures your aptitude for logical reasoning, not your ability to understand truth-functions. Consequently, the LSAT emphasizes inductive reasoning (e.g. reasoning from a sample, probabilistic reasoning, analogical reasoning), logical fallacies, certain types of syllogistic logic, and - of course - conditional reasoning. You will never need to construct propositional proofs, know what a "truth table" is, or understand modal logic. I can count on one hand the number of LR problems over the past year requiring you to setup "some" and "most" diagrams. Ten years ago, such questions were much more prevalent than they are today.
At their worst, college-level courses in formal logic will teach you to approach argumentation from a perspective that is too formalistic for the purposes of the exam, and not as attuned to the vagaries of language that test-makers have learned to exploit. You may develop a propensity to diagram every argument you see, which is downright counterproductive. It can slow you down, make you focus on the wrong task, and ultimately decrease your efficiency. For instance, just because many of the arguments on the LSAT can be represented using Venn diagrams does not mean they should be represented in this manner. Accuracy need not come at the expense of efficiency. We teach you how to do both. Most college-level logic courses do not.
They were never meant to.