Human Nature and the Many Definitions of the Words "Genes" and "Environment"
It is [...] in the very nature of genes that they are not turned on all the time but are expressed and regulated by a variety of signals. These signals in turn may be triggered by a variety of inputs, including temperature, hormones, the molecular environment, and neural activity.21 Among the environmentally sensitive gene-expression effects are those that make learning itself possible. Skills and memories are stored as physical changes at the synapse, and these changes require the expression of genes in response to patterns of neural activity.
These causal chains do not, however, render the nature-nurture distinction obsolete. What they do is force us to rethink the casual equation of ‘nature’ with genes and of ‘nurture’ with everything beyond the genes. Biologists have noted that the word ‘gene’ accumulated several meanings during the twentieth century.22 These include a unit of heredity, a specification of a part, a cause of a disease, a template for protein synthesis, a trigger of development, and a target of natural selection.
It is misleading, then, to equate the prescientific concept of human nature with ‘the genes’ and leave it at that, with the implication that environment-dependent gene activity proves that human nature is indefinitely modidiable by experience. Human nature is related to genes in terms of units of heredity, development, and evolution, particularly those units that exert a systematic and lasting effect on the wiring and chemistry of the brain. This is distinct from the most common use of the term ‘gene’ in molecular biology, namely, in reference to stretches of dna that code for a protein. Some aspects of human nature may be specified in information carriers other than protein templates, including the cytoplasm, noncoding regions of the genome that affect gene expression, properties of genes other than their sequence (such as how they are imprinted), and cross-generationally consistent aspects of the maternal environment that the genome has been shaped by natural selection to expect. Conversely, many genes direct the synthesis of proteins necessary for everyday metabolic function (such as wound repair, digestion, and memory formation) without embodying the traditional notion of human nature.
The various concepts of ‘environment,’ too, have to be refined. In most nature-nurture debates, ‘environment’ refers in practice to aspects of the world that make up the perceptual input to the person and over which other humans have some control. This encompasses, for example, parental rewards and punishments, early enrichment, role models, education, laws, peer influence, culture, and social attitudes. It is misleading to blur ‘environment’ in the sense of the psychologically salient environment of the person with ‘environment’ in the sense of the chemical milieu of a chromosome or cell, especially when that milieu itself consists of the products of other genes and thus corresponds more closely to the traditional notion of heredity. There are still other senses of ‘environment,’ such as nutrition and environmental toxins; the point is not that one sense is primary, but that one should seek to distinguish each sense and characterize its effects precisely.
A final reason that the environment dependence of the genes does not vitiate the concept of human nature is that an environment can affect the organism in very different ways. Some aspects of the perceptual environment are instructive in the sense that their effects are predictable by the information contained in the input. Given a child who is equipped to learn words in the first place, the content of her vocabulary is predictable from the words spoken to her. Given an adult equipped to understand contingencies, the spot where he will park his car will depend on where the No Parking signs are posted. But other aspects of the environment, namely, those that affect the genes directly rather than affecting the brain through the senses, trigger genetically specified if-then contingencies that do not preserve information in the trigger itself. Such contingencies are pervasive in biological development, where many genes produce transcription factors and other molecules that set off cascades of expression of other genes. A good example is the Pax6 gene, which produces a protein that triggers the expression of twenty-five hundred other genes, resulting in the formation of the eye. Highly specific genetic responses can also occur when the organism interacts with its social environment, as when a change of social status in a male cichlid fish triggers the expression of more than fifty genes, which in turn alter its size, aggressiveness, and stress response.23 These are reminders both that innate organization cannot be equated with a lack of sensitivity to the environment, and that responses to the environment are often not specified by the stimulus but by the nature of the organism.
21 Marcus, The Birth of the Mind; Ridley, NatureVia Nurture.
22 Ridley, Nature Via Nurture; Richard Dawkins, The Extended Phenotype: The Gene as the Unit of Selection (San Francisco: W. H. Freeman & Company, 1982); Seymour Benzer, “The Elementary Units of Heredity,” in A Symposium on the Chemical Basis of Heredity, ed. William D. McElroy and Bentley Glass (Baltimore: Johns Hopkins Press, 1957).
23 Russell Fernald, “How Does Behavior Change the Brain? Multiple Methods to Answer Old Questions,” Integrative Comparative Biology 43 (2003): 771–779.
Quoted on Sun Sep 22nd, 2013