One clear tendency is reciprocity. People are, of course, often self-interested -- but in many experiments, people will sacrifice their own interests when they believe others will also sacrifice, for the common good. People will also retaliate against what they see as wrongs, even when the retaliation makes them worse off, a negative kind of reciprocity. People do often act in their own self-interest, too, and different individuals act differently. But on the average, human behavior is not pure self-interest, but a mixture of self-interest and reciprocity.
This should not be very surprising. Generations of human beings and prehuman beings lived by hunting and gathering, and to survive in a hunting-gathering culture one has to cooperate and share. In hunting-gathering cultures, one hunter will get a catch one day, and another will get a catch another day, and the one who has been lucky on a given day shares with the others -- a form of reciprocity. Hunters and fishermen often have to work together, one doing one job and another doing a different job -- another kind of reciprocity. Putting up a temporary or a permanent shelter, self-defense, and many other survival-critical activities of early humans and prehumans require working together and reciprocity. Is it surprising, then, if the human mind has evolved with an innate ability to perceive and act on reciprocity, as it has evolved with an innate ability to learn language? Anyway, that is what the experimental evidence tells us.
The human tendency to reciprocity is not only a fact -- it is a value. What we do reveals our values. When one person makes a sacrifice for the common benefit, and another responds with a similar sacrifice, each is showing a commitment to the value of common sacrifice for the common good. And when one person refuses to consider the common good, and another responds by some retaliation that leaves the retaliator even worse off, the retaliator is showing his commitment to the value that one ought to consider the common good, and that to fail to do so is wrong. If these values are rooted in our genes and our evolution, not just in our subjectivities, that makes them all the stronger and more important.
Self-interest is a part of human nature, too, as it is part of the nature of all animals. And self-interest is not a bad thing in itself. Material well-being is a good thing, and it is good that a person should have what she wants to have, rather than what she doesn't want to have, if other things are not equal. Reciprocity comes into play when other things are not equal -- when she has what she wants by taking it from him, or when the way for both to have what they want is for each to make a special effort that will give no reward in itself, unless the other person matches it reciprocally.
Any ideal social system has its own values, assumptions, forms of organization, and intended outcomes. Ideal capitalism, for example, values only material well-being, assumes human beings are strictly self-interested and utterly rational, prescribes profit-maximizing enterprises and markets as organizational forms, and intends efficient allocation of resources as its outcome (as portrayed in neoclassical economics). As we sketch the values, assumptions, forms and intended outcomes of the Cooperative Commonwealth, we will need to refer to reciprocity often.