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Nature-Nurture Controversy - GENES AND BEHAVIOR

trait theory famous psychologists genetic twins studies human

Colloquial term for the two views of human development, one emphasizing heredity and the other environment.

The nature-nurture controversy is an age-old dispute among behavioral psychologists, philosophers, theologians, and theorists of consciousness as to the source of the creation ofhuman personality: Does it develop primarily from biology (nature), or from the environments in which we are raised (nurture)? People have been pondering the role of nature and environmentsince the time of Hippocrates (c. 460-c. 377 B.C.). He, for instance, linked human behavior tofour bodily fluids, or humors: yellow bile, blood, black bile, and phlegm. Hippocrates classifiedpersonalities into four types related to these four humors: choleric (yellow bile), or hot-tempered; sanguine (blood), or confident; melancholic (black bile), or moody; and phlegmatic, or slow totake action.

Unlike Hippocrates, the philosopher John Locke (1632-1704), whose ideas were aprecursor to behaviorism, believed that behaviors were externally determined. Similarly, thephilosopher Jean-Jacques Rousseau (1712–1778) theorized that people were born essentiallygood, and that positive aspects of the environmental contribute to the development of behavior. Locke believed that people were born essentially blank, like a blackboard, and who they "became" was entirely the result of their experiences.

The first scientist of the modern era to seriously consider the genetic and environmentaleffects in personality development was Sir Francis Galton, a wealthy British scientist. Hedabbled in the arts and sciences but became primarily interested in what we today call genetics after his cousin, Charles Darwin, published The Origin of the Species in 1859. He was fascinated by the idea of genetic pre-programming and-sought to uncover the ways in which humans are predestined. Many of his experiments were eccentric and illconceived, but his contributions to the field are still considered vital. His studies, curiously, led to the development of the science of fingerprinting and to the concept of the word association test. He also coined the term "eugenics" and believed that science would one day be able to direct, with absolute precision, the development patterns of human evolution. Taking the other position in this early debate was John Watson, the eminent behaviorist who once made the outlandish claim— which he later modified—that he could turn babies into any kind of specialist he wanted.

Over the years, much research has been done in the nature/nurture controversy, and today nearly everyone agrees that both nature and nurture play crucial roles in human development. This outlook has come to be known as interactionism and is the dominant system of belief among biologists, psychologists, and philosophers nearly everywhere.

Much of the research in the late 20th century has focused on twins who were separated at birth. In studying such pairs, psychologists can be relatively certain that any behavior the twins share has a genetic component, and those behaviors that are different have environmental causes. There are many famous cases of twins separated at birth being reunited later in life to find that they have many things in common. One of the most striking studies of twins, reported in a 1995 New Yorker article, was conducted by Thomas Bouchard, a professor of psychology at the University of Minnesota and founder of the Center for Twin and Adoptive Research. The twins, Daphne Goodship and Barbara Herbert, had been separated at birth and sent to economically different areas of London. The article's author, Lawrence Wright writes, "When they finally met, at King's Cross Station in May of 1979, each was wearing a beige dress and a brown velvet jacket…. Both had the eccentric habit of pushing up theirnoses, which they called 'squidging.' Both had fallen


GENES AND BEHAVIOR


Is a child's athletic ability inherited, or simply a product of training? If one parent hasschizophrenia, will his child acquire the disease? The genetic foundations of behavior are studied by behavior genetics, an interdisciplinary science which draws on the resources of several scientific disciplines, including genetics, physiology, and psychology. Because of the nature of heredity, behavior geneticists are unable to assess the role played by genetic factors in an individual's behavior: their estimates by definition apply to groups. There are 23 pairs of chromosomes in each human cell (a total of 46 chromosomes-each with approximately 20,000 genes). Genes from both members of a pair act in concert to produce a particular trait. What makes heredity complex and extremely difficult to measure is the fact that human sperm and eggs, which are produced by cell division, have 23 unpaired chromosomes. This means that one half of a person's genes comes from the mother, and the other half from the father, and that each individual, with the exception of his orher identical twin, has a unique genetic profile.

Scientists are working on the Human Genome Project recently finished mapping anestimated 100,000 genes in the human DNA. They have been able to identify genes responsiblefor a variety of diseases, including Huntington's disease, Down syndrome, cystic fibrosis, Tay-Sachs disease, and a number of cancers. Genetic information about a particular disease constitutes a crucial milestone in the search for a cure. For example, phenylketonuria (PKIU) is a disease caused by a recessive gene from each parent; PKU's genetic basis is clearly understood. A child with PKU is unable to metabolize phenylalanine, an amino acid found in proteins. Thephenylalanine build-up afflicts the central nervous system, causing severe brain damage. Because the genetic processes underlying PKU are known, scientists have been able to develop a screening test, and thus can quickly diagnose the afflicted children shortly after birth. When diagnosed early, PKU can be successfully controlled by diet.

While genetic research can determine the heritability of a some diseases, the genetic foundations of behavior are much more difficult to identify. From a genetic point of view, physical traits, such as the color of a person's hair, have a much higher heritability than behavior. In fact, behavior genetics assumes that the genetic bases of an individual's behavior simply cannot be determined. Consequently, researchers have focused their efforts on the behavior of groups, particularly families. However, even controlled studies of families have failed to establish conclusive links between genetics and behavior, or between genetics and particular psychological traits and aptitudes. In theory, these links probably exist; in practice, however, researchers have been unable to isolate traits that are unmodified by environmental factors. For example, musical aptitude seems to recur in certainfamilies. While it is tempting to assume that this aptitude is an inherited genetic trait, it would bea mistake to ignore the environment. What is colloquially known as "talent" is probably a combination of genetic and other, highly variable, factors.

More reliable information about genetics and behavior can be gleaned from twin studies.When compared to fraternal (dizygotic) twins, identical (monozygotic) twins display remarkable behavioral similarities. (Unlike fraternal twins, who develop from two separate eggs, identical twins originate from a single divided fertilized egg.) However, even studies of identical twins reared in different families are inconclusive, because, as scientists have discovered, in many cases, the different environments often turn out to be quite comparable, thus invalidating the hypothesis that the twins' behavioral similarities are entirely genetically determined. Conversely, studies of identical twins raised in the same environment have shown that identical twins can develop markedly different personalities. Thus, while certain types of behavior can be traced to certain genetic characteristics, there is no genetic blueprint for an individual's personality.

Twin studies have also attempted to elucidate the genetic basis of intelligence, which, according to many psychologists, is not one trait, but a cluster of distinct traits. Generally, these studies indicate that identical twins reared in different families show a high correlation in IQ scores. No one questions the genetic basis of intelligence, but scientists still do not know how intelligence is inherited and what specific aspects of intelligence can be linked to genetic factors.


down the stairs at the age of fifteen and had weak ankles as a result. At sixteen, each had met at a local dance the man she was going to marry. The twins suffered miscarriages with their first children, then proceeded to have two boys followed by a girl. And both laughed more than anyone they knew…. Neither had ever voted, exceptonce, when she was employed as a polling clerk."

Twin researchers, buoyed by stunning accounts like this, have been boldly asserting that nature determines who we are to a far greater degree than nurture. But twin research has its critics. One commonly pointed out flaw in twin research is that twins often mythologize, i.e., imagine or manufacture stories about, their shared characteristics. Also in dispute is how "different" the environments really are. Because adoption agencies screen applicants, families generally have certain shared socioeconomic characteristics. In addition, little research has beenconducted on "disconfirming evidence," that is, to ask the question, "Are there twins who show no remarkable similarities?" The nature-nurture controversy is far from settled.

Further Reading

Bouchard, Thomas. "Genes, Personality, and Environment." Science (17 June 1994): 1700.

Cohen, Jack, and Ian Stewart. "Our Genes Aren't Us." Discover (April 1994).

Cowley, Geoffrey. "It's Time to Rethink Nature and Nurture." Newsweek (27 March 1995): 52-53.

Gallagher, Winifred. "How We Become What We Are." Atlantic Monthly (September 1994): 39-55.

Wright, Lawrence. "Double Mystery." New Yorker (7 August 1995): 45-62.

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