What are the most common reasoning errors and why do we make them so often?

We think we want to buy the sweetest melon on the market: what is the most logical method that would first come to mind? In theory, taste them all. Since we cannot do this, we use a more “classic” system, that is, we smell, touch, etc. We use “broad” rules that could work, although we are aware that they cannot guarantee the success of the enterprise. The same happens for other types of reasoning: we approximate by necessity the information we have and, sometimes, we incur systematic errors in reasoning and evaluation. Reasoning errors (also called cognitive distortions) are automatic thoughts which, like “mental shortcuts”, lead us to interpret situations in a more rapid but at the same time distorted way.

This happens because it frequently happens that we find ourselves having to take decisions in situations of uncertainty or with limited information. However, we must be careful to attribute reasoning errors toirrationality of those who carry them out: the human brain it evolved trying to optimize the way it works and sometimes reasoning errors are linked to the rational process itself and we can all be victims of it, as we will now see with the examples of inductive and deductive reasoning errors.

Errors in inductive reasoning

An example of inductive reasoning is the diagnosis by doctor: given certain symptoms, it is likely that a patient has a certain pathology. The inductive reasoningtherefore, in summary, is what seeks to establish a universal law starting from individual particular cases.

In inductive reasoning, while we establish how much it can be likely fact, what Tversky and Kahneman call “heuristics”, i.e. fast mental procedures that unconsciously simplify the reasoning process. Heuristics can lead to errors because, when looking for a solution, they use “mental shortcuts” without exploring all alternative and possible paths.

Here are some inductive reasoning errors.

Tendency towards over-alternation or gambler’s fallacy

This cognitive distortion leads us to incorrectly estimate the probability of something happening based on events that have occurred previously. For example, if tossing a coin comes up “heads” 7 times I expect that the eighth time it will most likely come up as “tails”. In reality, however, the probability of a tails coming up when a coin is tossed is always 50%.

Availability heuristic

It is the tendency to believe that certain events are frequent or probable when they are very vivid in our mind. For example, believing that when we are in a hurry it increases the probability of finding red traffic lights.

Accommodation heuristics

It is the tendency to be influenced by previously provided information. For example, when we correct our incorrect estimates, we still tend not to deviate too much from the initial ones.

Errors in deductive reasoning

Deductive reasoning starts from one or more premises and comes to one conclusion without the need to resort to concrete data. Typical deductive examples are the conditional reasoning and that syllogistic. let’s see them

Conditional reasoning

Conditional reasoning is based on “if… then”. An example is: «If it is spring there are swallows». From numerous research, it emerges that individuals are able to follow this logical rule with ease in the case in which if the premise is true the consequence is true: if it is true that it is spring, it is true that there are swallows. However, one encounters difficulties in following the opposite reasoning, that is, “If there are no swallows then it is not spring”.

To understand better, let’s try to do theWason experiment from 1966:

We have before us 4 cardsboth with one letter on one side and a number on the other. What cards should you turn for to verify if this rule has been respected: «If a card has a vowel on one side, it has an even number on the other»? To give the right answer, the as few cards as possible.
In most cases, subjects turn the E, or the E and the 4.

• AND = correct choice but not sufficient: if on the other side there is an even number, I confirm the rule; if there is an odd number, I disconfirm it.
• AND And 4 = wrong choice: turning 4 is redundant. If there is a vowel behind the 4, I confirm the rule, if there is a consonant I get no information (the rule says nothing about consonants)

What is the right answer? Let us remember that we must verify the validity of the initial statement. And so we have to turn the E and the 7: if there is a consonant behind the 7, the rule remains valid, if there is a vowel, it is falsified. Only the 4% of subjects decides to turn over the cards E and 7: most people do not recognize the forgery process as useful. Why?

Why does the so-called “fallacy of the statement of the consequent”: we choose card 4 because we have a tendency to attribute symmetry to the conditional premise. In other words, the condition “if there is a vowel, then there is an even number” leads by symmetry to believe that “if there is an even number, then there is a vowel”.

Syllogistic reasoning

By syllogistic reasoning we mean a consequential speech which starts from certain premises and arrives at logical conclusions. Even here, however, we can come across some logical errorsfor example on the content.

Syllogism 1:

All men are living beings.
All Italians are men.
All Italians are living beings.

Syllogism 2

All cats have horns.
All animals are cats.
All animals have horns.

According to the rules of logical construction, both syllogisms are validbut from the point of view of reality, syllogism 2 is not acceptable compared to 1, because it is false in content.