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Posted
October 13, 2005

Trust and Reciprocity: A Platform for Evolutionary Success

When it comes to human nature, it's hard to generalize. Research shows that 25% of people act exclusively out of self-interest, while others are motivated in part by altruism.

Were all the artifices of civilization stripped away from Human Nature, what would be left? Is there an irreducible essence of what it means to be human? There are really no satisfactory answers to these kinds of questions. The problem is not that humankind did not evolve in a vacuum, but rather human beings are born incomplete. There is a deep evolutionary expectation that, as humans interact with other human beings and their environment, certain developmental responses are triggered. Some are predetermined, and some are not. Evolution counts on the external world to help carry the burden of development and differentiation — what the philosopher and biologist, Andy Clark calls the “scaffolding.”

Clark argues that there is insufficient information to completely specify the full development of the human body. Hence, there is no way that DNA itself could fully specify all the detail for the full development of a human being. Rather evolution makes extensive use of regularities in the environment — the scaffolding — to provide the requisite information to complete the growth of a full human being. Change the scaffolding and you change the human being.

So much for any effort to find a single definition of Nature. There is no essence of Human Nature — and the ideal of the Noble Savage is a fiction. Even “nature versus nurture” over-simplifies reality. Life co-evolves with its environment, with nature and nurture each reflecting and embedding the other.

Historically, this drama has taken place in the context of small tribal units. Some 95 percent of human history has been spent in small hunting and gathering bands where individual and group survival were interdependent. In this respect, early human society has direct parallels with primate society. It is irresistible, therefore, to ask: Were early humans like primates — selfish maximizers of their own self-interest? Were they a part of a winner-take-all culture, in which the powerful reigned over the weak? Or did early primates and by implication early humans, develop cooperative strategies — instances of what biologists call “altruistic reciprocity,” a willingness to take less for oneself in order to assist a relative or band member?

The answer is that primates do both. They can be extremely aggressive with one another, although it is generally moderated even with the alpha males. Smaller males form coalitions with other males and even alpha males, indicating that even among primates there is the ability to develop cooperative strategies of reciprocity. Food sharing and cooperative hunting are other examples of cooperation and the capacity to create “joint payoff” among primates.

Human beings are unique in the extent of their willingness to share food; it is a universal value among all human groups that food is shared among the nuclear and extended families. The sharing of food with strangers is also common among many traditional societies — hardly indicative of a singular preference for self interest.

So strong are the fitness values of reciprocity and trust that they have become embedded as a permanent function the brain. Human beings receive positive neurotransmitters, that is, chemical, rewards, for engaging in reciprocity. A significant part of the brain, the Nuclear Caudate, is dedicated to thought and behaviors associated with trust.

Trust is not simply a cultural or social construct, but actually a kind of circuit of neural responses that look for and reward trust and reciprocity. It is no less “fuzzy” than self-interest or aggression. In evolutionary biology, one rule of thumb is that if some behavior or emotion offers a real evolutionary advantage, it eventually becomes physically encoded in the brain. As the brain is a big consumer of energy and very stingy about what gets put into its neural “hardware,” emotionally encoded behaviors represent a major evolutionary endorsement. The fact that reciprocity and trust are two forms of social interactions that are encoded in the brain is strong evidence of their evolutionary importance and fitness value. They are as much a distinctive part of our Human Nature as are our hands or our bipedalism, larynx and language capacity. They are one of the unique adaptations that make us human.

There is a great deal of individual variation in how much people are willing to trust and reciprocate, however. An estimated 25 percent of human beings act exclusively in their own self-interest and are impervious to incentives to reciprocate. Based on the evolutionary and neurological evidence, one might reasonably surmise that the willingness to trust and reciprocate is a genetically linked trait that varies among populations, and has been selected for and against over the course of human history. Nonetheless, the fact that it is so prevalent across time and a spectrum of cultures and is so deeply encoded in the brain is indicative of its enormous and persistent fitness value.

In my next post: what all this means for economic theories of human behavior.