2005 : WHAT DO YOU BELIEVE IS TRUE EVEN THOUGH YOU CANNOT PROVE IT?

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Johnstone Family Professor, Department of Psychology; Harvard University; Author, The Better Angels of Our Nature
Psychologist, Harvard University; Author, The Blank Slate

In 1974, Marvin Minsky wrote that "there is room in the anatomy and genetics of the brain for much more mechanism than anyone today is prepared to propose." Today, many advocates of evolutionary and domain-specific psychology are in fact willing to propose the richness of mechanism that Minsky called for thirty years ago. For example, I believe that the mind is organized into cognitive systems specialized for reasoning about object, space, numbers, living things, and other minds; that we are equipped with emotions triggered by other people (sympathy, guilt, anger, gratitude) and by the physical world (fear, disgust, awe); that we have different ways for thinking and feeling about people in different kinds of relationships to us (parents, siblings, other kin, friends, spouses, lovers, allies, rivals, enemies); and several peripheral drivers for communicating with others (language, gesture, facial expression).

When I say I believe this but cannot prove it, I don't mean that it's a matter of raw faith or even an idiosyncratic hunch. In each case I can provide reasons for my belief, both empirical and theoretical. But I certainly can't prove it, or even demonstrate it in the way that molecular biologists demonstrate their claims, namely in a form so persuasive that skeptics can't reasonably attack it, and a consensus is rapidly achieved. The idea of a richly endowed human nature is still unpersuasive to many reasonable people, who often point to certain aspects of neuroanatomy, genetics, and evolution that appear to speak against it. I believe, but cannot prove, that these objections will be met as the sciences progress.

At the level of neuroanatomy and neurophysiology, critics have pointed to the apparent homogeneity of the cerebral cortex and of the seeming interchangeability of cortical tissue in experiments in which patches of cortex are rewired or transplanted in animals. I believe that the homogeneity is an illusion, owing to the fact that the brain is a system for information processing. Just as all books look the same to someone who does not understand the language in which they are written (since they are all composed of different arrangements of the same alphanumeric characters), and the DVD's of all movies look the same under a microscope, the cortex may look homogeneous to the eye but nonetheless contain different patterns of connectivity and synaptic biases that allow it to compute very different functions. I believe this these differences will be revealed in different patterns of gene expression in the developing cortex. I also believe that the apparent interchangeability of cortex occurs only in early stages of sensory systems that happen to have similar computational demands, such as isolating sharp signal transitions in time and space.

At the level of genetics, critics have pointed to the small number of genes in the human genome (now thought to be less than 25,000) and to their similarity to those of other animals. I believe that geneticists will find that there is a large store of information in the noncoding regions of the genome (the so-called junk DNA), whose size, spacing, and composition could have large effects on how genes are expressed. That is, the genes themselves may code largely for the meat and juices of the organism, which are pretty much the same across species, whereas how they are sculpted into brain circuits may depend on a much larger body of genetic information. I also believe that many examples of what we call "the same genes" in different species may differ in tiny ways at the sequence level that have large consequences for how the organism is put together.

And at the level of evolution, critics have pointed to how difficult it is to establish the adaptive function of a psychological trait. I believe this will change as we come to understand the genetic basis of psychological traits in more detail. New techniques in genomic analysis, which look for statistical fingerprints of selection in the genome, will show that many genes involved in cognition and emotion were specifically selected for in the primate, and in many cases the human, lineage.