Whether there is more behind human behavior than can be explained solely by neural phenomena has been the subject of much time-worn scientific and philosophical debate. In regards to this question, two primary classes of alternative explanations come to mind: the human soul and the environment. The former of these involves a possible internal, individualistic force guiding behavior beyond the guides provided by the brain; many feel that the topic of the human soul is best left in the realm of the philosophical. Environmental influences on behavior, however, are quite pertinent to scientific investigations into the brain/behavior dichotomy. Whether and to what extent one’s environment effects one’s behavior, personality, even destiny is embodied by the widely publicized and highly politicized nature-nurture debate. Generally, those factors thought to come from nature are those that are inherited, and those thought to be nurture-bred are inculturated. It seems, then, that a discussion of neural control of behavior necessarily involves this nature-nurture question; the interplay between inherited brain structure and inculturated experience offers insight into the roots of behavior.
Vitalists and reductionists, empiricists and situationists, geneticists and sociologists all have something different to say about the degree to which the human genome specifies human traits. Many of these traits are behavioral, thus, these people also have much to say about the role of the brain in guiding behavior. It is estimated that sixty percent of human genes are dedicated to neurological development (5). This is an immense amount of genetic material, and, particularly during fetal development, but indeed throughout one’s life, it is all aimed directly at that heavy, fluid-bathed, beautifully sophisticated organ between the ears. What exactly do these roughly sixty thousand (there are around 100,000 genes in the human genome) genes specify? Some encode structural proteins which find their form in macrostructures such as the hypothalamus or the frontal cortex as well as microstructures such as neurons. Others of these genes specify functional proteins which comprise the classes of enzymes and hormones so essential for neural functioning.
That the large majority of an individual’s inherited genetic material stipulates pathways for brain development may not impress the nurturists in this debate. After all, the brain is also the body’s greatest recipient of ATP, oxygen, and many ions; it is simply a demanding organ, and understandably so, given all that it affords the rest of the body. The relationship between genes and the brain can be looked at in a different light, however. For not only is sixty percent of the genome dedicated to the brain, but one hundred percent of the brain was specified by the genome. This point cannot be stressed enough in the nature-nurture debate, as it has tremendous implications for the school of thought known as genetic behaviorism. The genetic-behavioral model argues that “genes are the most important factor affecting both our health and behavior, but that the way in which genes express themselves is influenced by the conditions in which we live” (4). In a neurobiological sense, this means that an environmental factor will only influence human behavior if an appropriate, genetically determined transmission point can be located in the brain.
Suppose, for instance, that genes A, B, and C encode neurons a, b, and c respectively*. Also suppose that, if expressed normally, neurons a, b, and c are networked in such a configuration that the nerve impulses between them are part of a crucial step in the pathway for perception of a certain color- yellow, for example. In this terribly simplified scenario, one’s entire experience of bananas, school buses, and #2 pencils (and the myriad of memories and cognitions that follow from such experience) are dependent on the presence of neurons a, b, and c. And neurons a, b, and c are, in turn, dependent on the expression of genes A, B, and C. It follows, then, that a person’s yellow-reality can be completely altered by the presence or absence of specific genetic material; genes are the gatekeepers of experience, they are the sieves through which we experience the world. Another weighty point on the genetic reductionists’ side of the debate is that while genes can change the way the environment effects an individual, it is much less common for the environment to alter one’s genes. For example, an individual with two mutant alleles for the gene which causes Huntington’s disease to manifest in mid-life has an altered experience of his environment due to his genetic make-up. However, no known environmental agent will spontaneously change a person’s genes to include mutant alleles for Huntington’s disease.
This line of reasoning begs the question: if all behaviors have their roots in the genome, how does one explain a neurological or psychological disorder such as anorexia which has no known single genetic correlate and is not known to be heritable? To answer this, some call upon the concept of polygeneity, that complex behaviors are bred from not one but many interacting genes. Others speak of the difference between genes <">predicting behavior and genes <">predisposing someone to behavior (3). A prediction situation is analogous to being homozygous for a particular trait (i.e. the genotype can be directly derived from the phenotype), while a predisposition situation is analogous to heterozygosity for the trait (i.e. the individual is merely a carrier of the mutant allele). Thus, a woman may be predisposed to develop breast cancer but has only one mutant BRCA1 gene, so appears normal. An individual, then, would be predisposed to anorexia if only some of the genes, which present together in their mutant form cause the expression of anorexia, were actually expressed. And anorexia could be predicted in this individual if all of the genes were present in their mutant form and the anorexia phenotype could be expressed. This is the way that some extreme genetic reductionists have to think about genes and behavior if they are to rationalize their stances in the nature-nurture debate.
Nurturists argue that neurbiologists will never be able to locate neural centers for such complex behaviors as forming a prejudice, enjoying a sunset, or mourning the loss of a loved one. Such behaviors, they posit, are regulated by the context of one’s experience, by an individual’s family, society, and culture. They are behaviors which cannot exist without the forces of custom, morals, and societal expectations. But what is a society, argue back the naturists, but a collection of genetic beings? And what, for that matter, is a moral but a programmed array of emotions- emotions which are essentially chemically based and dependent on appropriate neuronal receptors for activation? It has been said that humans can only experience a new emotion when there is an evolutionary change made to the genetic material (2). This implies that feelings (or at least the ability to feel) are heritable, and that my great-grandmother’s sadness, joy, and anger are manifest in my own sadness, joy, and anger. This gives a whole new meaning to what is largely thought of as the human collective experience.
While I do consider myself a genetic reductionist, I am inclined to postulate that the nature-nurture debate as such will never be fully reconciled. It seems that there are too many unknowns, too many things unknowable. Besides, there is always the “association is not causation” problem. Geneticists who propose a novel gene for behaviors such as homosexuality, alcoholism, or aggression are forced to acknowledge that external, environmental factors could be inducing the association between genotype and phenotype (behavior). So the problem is still the same: for whatever input comes through external instruments into our physical brains will still be subject to biased interpretations of which we are capable as human-mammals. “Our concepts of reality will always be shaped by our genetically-inherited mental models” (3). Subjecting myself to biased interpretations, though, I feel that an individual drawing breath each day in the context of many, nestled environments much akin to the Bronfenbrenner ecological systems theory **, behaves ultimately because of the brain and the genes from which it originates.
* Of course, this is an exaggeration of the gene to neuron ratio. While there are only 100,000 genes (60% of which are dedicated to neural development, and who knows how many to neurons themselves), there are an estimated 10^12 neurons in the human brain. Therefore, there must be many neurons encoded by a single gene.
** Bronfenbrenner, an American psychologist, developed this theory which views the child as developing within a complex system of relationships affected by multiple levels of the surrounding environment.