As we read the newspaper each day, most of us ask ourselves why people can't be nicer to one another, more helpful, more cooperative? And indeed, one could phrase the central normative question of the social sciences as what can be done to encourage people to be more cooperative: to work together against war, against the degradation of our climate, and for economic security for all?

But in direct contrast to this basically humanistic question, the central biological question in the study of human evolution is why humans are so inordinately cooperative as compared with our nearest primate relatives. In most primate groups, competition is the norm, and cooperation occurs mostly among kin, who share genes and so have a biological stake in one another's well-being, or else among a few individuals who cooperate reciprocally. But humans truly sacrifice for one another on a very large scale and not just with kin - we donate blood for unknown others, we risk our lives in war for our countrymen in general, and we make sure that our less fortunate compatriots have enough food and medical care to survive. On a daily basis, we obey all kinds of prosocial norms - from helping older people with their luggage on trains to allowing others into line in traffic - that we really don't have to. And when we do breach some prosocial norm, like not doing our part in a collective enterprise, we feel guilty or ashamed - a sure sign that Mother Nature has crafted us to be cooperative.

In this book, based on my Tanner Lectures at Stanford University, I review recent research, much of it from our laboratory, on the pro-social tendencies of young children and their nearest primate relatives, most often chimpanzees. There are two lectures. The first lecture is about young children's tendency to help others. For example, when 14-month-old infants observe a stranger having trouble fetching an out-of-reach object, or struggling to open a cabinet, or needing to move something but his hands are full, these infants - some barely walking, none talking, and all in diapers - quite often toddle over and help. Importantly, they do not help more often if they are directly rewarded, and indeed if one gives them material rewards for helping and then takes these rewards away, the infants subsequently help less - the reward seems to have actually undermined their natural, intrinsic motivation. In addition, even, prelinguistic 12-month-old infants will assist an adult by pointing to the hidden location of some object she is searching for - with no motivation to have it for themselves, but only to inform the adult helpfully of its location. It seems that Rousseau, not Hobbes, was right in the first place: humans are born helpful, or at least they become so very early in development before much active socialization and teaching has taken place. This conclusion is reinforced by the fact that our nearest primate relatives, chimpanzees, will also help others in some (but not all) of these situations as well, without any human socialization at all.

The second lecture is about working together collaboratively toward a mutually beneficial outcome. In this case, perhaps surprisingly, children and chimpanzees are more different. Thus, at two years of age, when their cognitive abilities for understanding the physical world of space, objects, and causality are still identical with those of chimpanzees and other apes, young children already are able to collaborate by forming shared goals and dividing the labor among participants in species-unique ways. For instance, when two chimpanzees pull in a long board with food on each end, they work together well and each takes the food on their end. But when the food is clumped in the middle of the board, they have difficulty cooperating, presumably because they are anticipating the competition, if not the fight, over the food at the end. In contrast, young children are not affected by whether the food is distributed on the ends of the board or clumped the middle. They almost always take equal shares in either case, and cooperation continues because they both trust that they will be able to work out a fair division of spoils at the end. We have even found that in a cooperative task if one child's reward suddenly appears first, she nevertheless feels a commitment to finish the collaboration so that the other gets his reward as well - whereas chimpanzees do not seem to feel this same commitment. Human children are uniquely equipped to engage with others in many different kinds of "shared intentionality".

So what happens to these angelic little toddlers? How do they turn into the adults who do all of the not-so-nice things that we do to one another? Part of the answer, of course, is that at the same time they are doing all of these nice things, young children are also doing plenty of selfish or even mean things as well. We humans are biologically evolved to look out for our own interests as well as those of others. Another part of the answer is that people who are cooperative and helpful indiscriminately all of the time will end up getting taken advantage of by others, as Darwin already noted. Computer models of the evolution of cooperation show that indiscriminate cooperators almost always end up losing against "cheaters" who accept helpful acts from others but do not reciprocate.

The basic problem of the evolution of cooperation is thus that nice guys get taken advantage of, and therefore they must get some kind of compensation for their sacrifices. More than any other primate species, humans have overcome this problem through a variety of mechanisms such as reciprocating cooperative acts (underlain by feelings of gratitude and obligation), forming reputations of others and the self as cooperators and caring about these reputations (sometimes underlain by feelings of pride), and creating prosocial norms about good behavior that everyone in the group will enforce on others through disapproval, if not punishment, and will enforce on themselves through feelings of guilt and shame. We have found that by 3 years of age young children not only follow social norms, but they enforce them on others. If they see a child threatening to steal something from another or destroy her precious picture, they intervene and say "No. You can't do that. It's not right." They even engage in this normative enforcement if someone is breaking the rules of a game. They also show guilt if they break someone's favorite toy they were told not to touch.

Humans are not prosocial angels - neither children nor adults - but they do have strong prosocial tendencies that compete with their selfish tendencies. As history moves us forward, we do not know which tendency will win the day, but we have a lot to learn about the process from young children and our nearest primate relatives. Perhaps that will help.