In the new definition of reasoning, what reasoning is, is the process of finding reasons (justifications, rationalisations etc) for decisions made and/or actions taken. First comes the decision, then the reasons. It's always this way around for us, and unless someone enquires, we may not even have reasons for things we do, think or say. Unconscious processes guide all of our actions, but we are equipped to explain them to others if required. But we do this in a post hoc manner: reasons come after the fact and on demand.
Unfortunately, humans have biases in this department. For example, we stop searching when we find any plausible reason, we don't keep searching for the best reason. Unless we are arguing with someone who shoots down our reasoning. Reasoning is a group activity and solo humans don't do it very well.
When we don't have strong intuitions about a decision, it still better to go with our gut. When we stop to reason about a decision it drives us towards decisions that are easier to justify. But in the long run, such reasoned decisions turn out to be less satisfying.
One of the reasons we do this is to appear rational to our peers. This is a very important for humans. We are social and in the modern world appearing to be rational is an important aspect of group membership. Rational is defined locally, however. What is rational for the girl guides, is not rational for the Tory Party or the Hell's Angels or my family.
Rationality is being able to offer reasons for actions and decisions that one's peer group accept as being rational.
Sometimes when trying to fit into our social group we make decisions that seem less than rational to an outsider. "Would you jump off a cliff if they told you to?" Anyone who has heard this in earnest will know what I mean. As if happens my paralysing fear of falling kept me from jumping off cliffs, but it was a situation I faced in real life and yes, had I not been phobic, I would have jumped. I wanted nothing more than to jump off that cliff and be one of the gang. I did other brave things. Just don't ask me a jump of a curb, let alone a cliff. Although I was always fascinated by space, I knew at a very young age that I did not want to be an astronaut for this very reason.
An outsider may see this as irrational. But as human beings, it may be more rational for us to do some mildly irrational things that assure us of group membership because group membership is a long term survival mechanism. We evolved to live in groups.
While making irrational decisions may be suboptimal, losing my social status, let alone being ostracized, is a catastrophe. So there is a delicate balance that we all know. We allow ourselves to be pressured into conforming because instinct tells us that acceptance is more important than rationality. And this is true.
Or it was true 12,000 years ago in our ancestral environment. In that milieu, living as hunter-gatherers, satisfying the expectations of our peers, was probably a good rule of thumb for life. More so when we consider that our "peers" included the older more experienced members of the tribe.
So yes, people succumb to peer pressure. They behave in atrocious ways. But at the time, in their milieu, it may have been the rational thing to do, no matter how ugly it seems to us now. Until you're in the situation, you don't know how you'll react. This is why surveying someone's opinion of how they would react is meaningless. What we do in crucial situations cannot be predicted, especially by ourselves. Asking people about the trolley problem (where you can rescue 5 people by killing 1) for example is meaningless. No one knows what they would do in that situation.
All we can do is imagine that we have done something and how easily we can justify it. If we are further asked to explain ourselves, it will often change our answer, since we have to say the reasons out loud and watch the reactions of the person asking the questions. We get a better idea of how the justifications sound and we chose the best justification, which tells us what action we might do in that situation. I'd be willing to bet that there is no long term relationship between what we say we might do in these extreme hypothetical situations and what we actually do when it comes down to it. Although in more realistic scenarios that we actually have experience of, we can turn to that experience to guide us.
So rationality is not what we were taught. It is not what philosophers have classically defined it to be. Most solo humans are poor at reasoning and only reason well when arguing against someone else's proposed proposition. Reasoning certainly uses inference to produce reasons, but it does not help us find truth or make better decisions. It may help us convince people that the decision we have already made is the only decision they could have made, or the best one, or it may help us describe why someone else's decision is the worst one.
The problem with the classical view of rationality and reasoning is that it is completely at odds with the empirical evidence. It is a fiction maintained in spite of the evidence. The classical view of rationality and reasoning is so far past its use-by date that it approaches being intellectual fraud or hoax. What is actually happening is a lot less grandiose, a lot more banal, but it is what it is. We are what we are. Living a fantasy is the epitome of irrationality.