پس زایی لکنت زبان

This paper examines the data on which the Demands and Capacities Model (DCM) is based with the purpose of identifying areas where future research might determine consilience among genetic influences at the physiological, behavioral, and cultural levels. The determination of consilience across different levels would tend to validate the genetic influence on stuttering, but more importantly it would also sharpen the focus of researchers interested in the various possible expressions that genetic influences might have and the way in which they influence the development of the disorder. Educationalobjectives: The reader will (1) learn about the distinction, as outlined in the DCM, between environmental/contextual influences on stuttering development and intrinsic/genetic influences on stuttering development; (2) learn about the concept of consilience and its usefulness in conferring validity on parallel constructs at the physiological, behavioral, and cultural levels of stuttering theory; and (3) be able to identify potential areas for research that might help in refining our understanding of the genetic influences on stuttering development.

During the past 10 years, there has been much interest in genetic explanations for stuttering. This paper explores a number of possible genetic influences, but from a broader perspective. In 1990, after many public presentations, my colleagues and I wrote and published the Demands and Capacities Model (DCM) of stuttering development (Starkweather, Gottwald, & Halfond, 1990). This model of stuttering development had been inspired by E.O. Wilson’s book Sociobiology ( Wilson, 1975), which delineated a number of the ways in which genetic predisposition in animals could interact with environmental events to produce both individual and social expressions of behavioral patterns. It seemed to us that an examination of the interactions between genetic and environmental influences on stuttering development would also be useful. A thorough examination of the literature on stuttering then extant contained many findings that shed light on (1) the specific environmental/contextual1 events that made stuttering behavior in a child more or less severe and (2) the specific organismic traits that hindered or helped the development of fluent speech. Further, the data suggested that there were four areas of functioning — motoric, linguistic, emotional, and cognitive — in which influences on the development of fluency in a child’s speech could be identified. For some variables, there were few data, and it was necessary to lean on the accumulated clinical wisdom of speech pathologists who have worked with young children. For other variables, the data from studies of adults could be applied to children only with considerable caution. But there was enough information, either solid or inferred, to create a model of the development of stuttering. The sources of influence for this model are listed in Appendix A. It seemed obvious, since these influences were not mutually exclusive, that they could simultaneously coexist and therefore summate to produce a net effect on the child. It also seemed likely, if not obvious, that each child had a threshold of influence above which the net effect of these influences could create or inhibit disfluency. Only one assumption needed to be made: that the more disfluent a child’s speech was, the more opportunities there would be for reactions of struggle and avoidance to develop. Thus we could show at least the outline of a model of the influences on stuttering development. Further, clinicians were able to obtain sufficient guidance from the model, incomplete though it was, to develop a set of principles with which to assess individual families for the purpose of identifying the influences, both organismic and environmental, that were affecting a particular child’s fluency development. As a result of such assessments, clinicians were able to develop individualized plans for intervening with very young children, plans that proved to be remarkably helpful (Starkweather et al., 1990). The percentage of children who recovered after being treated with methods based on this model was well over 90%, substantially higher than the percentage that would be expected, even by the most optimistic reports, to recover by chance. Further, and even more compelling, the time taken for these children to recover was, on the average, only 8.6 weeks (Starkweather et al., 1990), whereas spontaneous recovery, when it occurred, seemed to be a matter of 6 months or so (Andrews & Harris, 1964 and Yairi & Ambrose, 1992). Since then, Wilson has published another book, Consilience (1998), which develops with far more breadth than in his previous book, the relation between genetics and human behavior. The word consilience, as Wilson uses it, means a unity of knowledge across different disciplines. To demonstrate this unity, Wilson shows relations among information taken from physics, chemistry, biology, psychology, sociology, anthropology, and economics. Thus, the microbiological functioning of cells is described, and knowledge at this level (physics, chemistry, biology) is shown to be coherent with knowledge of the behavior of organisms, both individually (psychology) and in groups (sociology, anthropology, economics). By relating genetic studies with behavior in the individual, and then by relating individual behavior with group behavior, Wilson shows that genes play an influential role in the development of culture. Indeed, Wilson shows that elements of the arts, religion, and ethics are related to anthropological and sociological data, which are, in turn, related to data at the physical and chemical levels. Thus knowledge, whether at molecular or molar levels, is shown to be based on similar inductions. When consilience is found between two sets of logical inductions it confers on both sets a more probable conformity with the truth ( Gove, 1986). Some concrete examples will perhaps help clarify the concept of consilience. Surely the simplest example is the following: there is a high prevalence of genetically transmitted lactose intolerance among people of Asian descent, primarily Chinese. This biological condition is closely related, for obvious reasons, to the behavioral fact that Chinese people drink very little milk, and eat little cheese or butter. Consequently, at the cultural level, there are few dairy products in any Chinese recipes. This example may seem trivial, but it illustrates the connections very well. But because the connections are so obvious and the facts so well known this example does not clarify how consilience can increase the validity of the influence of genetics on diet. A less trivial example concerns aggressive tendencies in some individuals. Such tendencies are probably influenced both by genetics at the biological level and environmental events at the psychological level in childhood, but it seems quite reasonable to assume that genetics plays a role in their expression in individuals. Individuals who grow up with such a tendency toward aggressive behavior would be likely to have values that correspond with their own feelings and therefore confer legitimacy on them, so they would be more likely to seek out professions, activities where those values are appreciated and acquaintances who act on them and shun the company of people whose values about aggression and violence are the opposite. As a result, an aggregate of aggressive people is likely to form a kind of culture, or subculture, in which these shared values are prevalent. The result of such a culture would be a medium in which aggression and violence are more readily accepted and can grow, and in which those who do not share these values are seen as enemies, or at least as “the other.” It seems likely, given such a scenario that groups of militias, or even terrorist groups, could develop and even flourish in these circumstances. The preceding example is of course a hypothesis, but it is a hypothesis of the same kind that we are interested in with regard to stuttering. A program of research designed first to establish a genetic link to aggressiveness, and then to test the genetic makeup of members of militia groups for the presence of this genetic pattern would, if positive results were found, demonstrate consilience at the levels of microbiology, behavior, and culture, and consequently confer validity on the hypothesis that genes are an important determiner in both aggressiveness and militia groups. Similarly, a program of research to establish a genetic link to stuttering has already been carried out, and it remains to test the genetic makeup of stutterers and the groups of individuals with whom they interact, i.e., their parents, siblings, and peers, indeed the public at large, to see if the predicted traits are found. Such a program, if it produced positive results, would establish consilience among the biological, psychobehavioral, and cultural levels of stuttering. Wilson begins by demonstrating that genetically influenced behavior patterns confer Darwinian fitness on organisms, including human beings. For example, he describes the role of altruistic behavior among family members — behaviors such as infant care/adoption by relatives, sharing of food among relatives, even self-sacrifice of one relative for others — as strengthening the fitness of the entire family and thus increasing the probability of survival for all family members. The patterns of distribution of altruistic acts suggest compellingly that genetic influence plays a role in their expression. This relationship of altruism to family survival demonstrates consilience between microbiology and behavior. He goes on to show that these same patterns of behavior are repeated in cultural codes of conduct. For example, in our society, the sharing of food or other resources with family members is an activity that is approved of; it is valued, and we feel good about ourselves when we behave in this way. The same can be said of self-sacrifice. We admire the brother who foregoes a university education so that his brilliant sibling can attend law school. Such stories are familiar to us, and consequently make the point of demonstrating consilience between genetics and ethics. This demonstrated consilience argues even more strongly that genetics plays a role in many levels of human life. Since the DCM attempts to organize the information on stuttering’s development at the behavioral level into intrinsic (genetically influenced) and extrinsic components, it would seem to be a healthy exercise to reexamine the DCM now to see if it may or may not conform to what is known about stuttering below the level of behavior, i.e., physiologically, and what is known about stuttering above the level of individual behavior, i.e., culturally. Establishing consilience between the physiological genetics and the behavioral genetics of stuttering, and/or between behavioral and cultural genetics of stuttering will validate further the genetics of the disorder. But since it is already well established that genetics plays a role in the prevalence of stuttering, a more important result of establishing consilience among the various levels of data will be a determination of areas of research that will identify more closely the relations of genetics to (1) the physiology of stuttering, (2) the behavior of stuttering, (3) the environments and contexts in which stuttering occurs (i.e., the stimuli associated with stuttering in individuals), and (4) the values placed on stuttering by modern society. Research in these areas will refine our understanding of the nature of the genetic influences on stuttering development.