Humans are animals, and therefore, have instincts. If the proceeding sentence offends you in any way, please stop reading now. The most commonly cited of these instincts are eat, procreation, and the so-called "fight or flight" reflex. Hence, these will not be discussed here.
Beyond this are several other instincts that are also obvious after some consideration. For instance, when babies are frightened they cry for help. This is hardly learned behavior as it is demonstrated too soon after birth for it have been learned. Also, the baby could not have learned it from a parent, as adults rarely bawl out when frightened, hungry, or wet. Granted the latter instance is usually due to a desire not to attract attention. Since this behavior is observed whether or not the child in question is in the presence of other children, it did not come from the parents, and could not have been invented it must therefore be instinctual. While infants do learn that when they cry their parents come, it is only because they already cry when something untoward happens that they can correlate the two.
There are many other autonomic reflexes that are instinctual; however, they lead to less amusing anecdotes. There are three specific instinctive behaviors I want to discuss. Namely, these are efficient action, the creation of communities, and gathering during perceived times of danger.
"What do you expect? People are lazy." I have heard this said many times before, and until recently I never really considered it. But now I always think to myself, "why do you say that like it was a bad thing?" Laziness is quite adaptive as is produces several reactions. First, while a lazy person will do everything they need to do they will neither look for more to do nor will they expend excess energy doing what they had to do. Therefore they have more energy saved for later, when it might be needed more. As such, laziness is simply a question of efficiency. Secondly, it makes exploitation more difficult. Exploitation requires massive expenditures of energy. A lazy person is unlikely to expend that energy unless they feel they must. In other words, unless a lazy person feels that their food, water, or safety is in jeopardy they will not expend the energy required for exploitation. Such is the case in civilization, where otherwise efficient humans believe that unless they work forty or more hours a week they will not be able to eat. This is a memic-illusion more than a fact, but it is yet a very real motivation. And, apparently, is quite sufficient to cause these persons to work.
When people gather in small groups without a previously determined social structure they invariably form communities. When a group of friends gets together for a night out, it is very rare that one person simply tells everyone else what to do in order to have a good time. More often either of a form of democracy or consensus is used. What is most intriguing about this is that democracy is only used if the group formalizes their decision. If the decision remains in formal then the group as a whole often discusses the options until an idea is reached that everyone in the group is happy with.
The power is out, everyone grabs their flashlights and makes their way to a central location. This happens so regularly as to be completely unremarked upon. The power goes out in a dorm, and two minutes later everyone is in the hall or in the floor lounge. The power goes out at home, and the entire household gathers in the living room. The power goes out in New York City, and everyone takes to the streets. Black-outs are simply a single example of a broader instinctual reaction. Studies have been done demonstrating this. The sociologists split their test group into two parts. The first was assured that the experiment was harmless, safe, and would not hurt. The second was warned that there would be pain and there was a risk of semi-permanent damage. Each group was given a questionnaire asking if they would like to wait their turns alone or together. Every member of the second group requested to wait with someone else. Many in the first group were fine waiting alone.
When the power goes out everyone does not think to themselves, "I should probably go outside now." Everyone simply grabs their light source, if they have one, and goes to a central meeting place. No signal, order, or decision is required. The power goes out, and people gather. It is only when members of that group do not gather that it becomes obvious that something is amiss. That person might feel uneasy, inordinately alone, or even fearful. This is quite regardless of whether there is any actual danger. Instincts relating to defense activate when there is a perceived threat, any actual danger being quite without standing. The historical antecedents of this instinct are easy to see. If a band of humans were in danger their best chance of survival was always when they were together. An individual human is always quite vulnerable, only in a group do people become dangerous.
So, if humans do better in groups, why do they seem to be so selfish? This comes from a misunderstanding of what selfishness really entails. A person will want what is best, not only for themselves, but also for those they consider kin. A person in a familial group, regardless of blood relation, will always work for not only their benefit but also the benefit of the group. Another way of looking at it is to remember that, in forager groups, the safety and prosperity of an individual is absolutely reliant on the safety and prosperity of the group; but, there is no benefit to support the safety and prosperity of other forager groups. In fact, expending energy providing for their safety and prosperity would be counter-productive. It would drain a person's own resources without benefit to themselves or their offspring. So, when humans evolved our instincts selfishness was a productive trait.
Ultimately the difficulty in understanding human instincts is that all instincts work invisibly. Instincts are, by definition, not conscious decisions, but rather an unconscious need. This need is not logical or sensible in the usual sense. While there is a distinct reason behind each instinct, otherwise they would not exist, they are not something that a person considers and then does. Rather suddenly that person feels that they have to act in a certain manor, and they do. This manor is the latest version of instincts dating to the first primordial ancestor.