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Why do humans reason massively incorrectly?

1. Because they confuse reasoning and rhetoric.2. Because their reasoning is most often constructed backwards.3. Because they constantly apply heuristics, or worse, dogmas.4. Because they notice that their status is sufficient to impose a decision, thus freeing them from the need to reason.

What is rhetoric?

It is the art of gaining the upper hand in a verbal duel. We are particularly attached to it in public, because it corresponds to positioning oneself above others in terms of social rank. In other words, in public, we generally seek not so much to be right as to have the upper hand.

What is a backward reasoning?

In correct reasoning, one conducts an exhaustive analysis of the situation, which may eventually reveal a conclusion that one discovers.
In backward reasoning, one starts from a conclusion that has been arbitrarily chosen, because it conforms to one's immediate interest or beliefs, and then constructs a biased reasoning afterward, consisting of selecting and articulating the elements that justify this conclusion, to create the illusion of reasoning stemming from an analysis.

What is a heuristic?

A heuristic is a method that allows one to find a not too bad solution to a complicated problem with far less effort than would be required to find the optimal solution.

Let’s take the example of the traveling salesman problem. A traveling salesman must visit prospects in, say, 100 cities, and is looking for the shortest route to achieve this. Finding the actual shortest route is not possible for this salesman, who will therefore adopt shortcuts (heuristics) to make the task easier.
A very simple heuristic could be to go from city to city, choosing at each step the closest remaining city. It is very easy to trace. However, it is far from optimal at the end of the journey.
Our salesman might then apply a second heuristic that involves going back to the last cities and deciding to visit them earlier, making the shortest possible detour in the journey at each step.

In our lives, we constantly apply heuristics. The problem is that the less competent we are in a domain, the more we tend to use simplistic heuristics. As a result, our natural functioning leads us to easily find poor solutions.

The interest of heuristics for the individual is twofold:  
1. They allow one to avoid the effort of reasoning, thereby saving time and energy.  
2. By using dogmas as heuristics, one generally improves their position in the game of alliances. Dogmas are often ineffective heuristics, or even completely erroneous ones, meaning they lead to very poor solutions to the original problem.

Go deeper

Refer to the question 'What is a human?' which provides another perspective on the low quality of our reasoning.

Refer to chapter 4 'The decision-making process, or the reign of the irrational' of the book From Capital to Reason.

↣ Wikipedia article regarding cognitive biases

↣ Wikipedia article concerning cognitive misery