Heredity Versus Environment

Many aspects of human characteristics (such as height and eye color) are largely genetically determined. Psychology researchers, however, tend to be interested in dimensions that are relatively less determined by genetics—traits that subject more to environmental influences, such as how a person feels, acts, and thinks. Given that the degree of genetic determination appears to vary from one dimension to another (e.g., spatial skills versus language acquisition), how can one determine the relative influences of heredity and environment for various human characteristics, and how can one understand the complex relationship between them?

For example, Javier has two biological daughters who share the same biological mother. Both are tall, well mannered, and musically inclined. Despite these similarities, the older child appears socially reserved and quiet, while the younger one, who was born into the same family environment, seems more outgoing. In addition, one of his children has been diagnosed with a learning disability while the other seems exceptionally well-functioning cognitively. How can these similarities and differences between the two children be explained? One may think, ''Well, Javier is tall and he is also a talented musician himself, so these girls must have gotten these 'good genes' from Javier. And he is quite strict when it comes to disciplining his children, so that explains their good manners. But why is the younger one so sociable—and what about her learning disability? Maybe she hasn't been read to as much as the older one has.'' In essence, hereditary influences and various environmental factors in these children's lives are being weighed and analyzed in explaining the characteristics of these children.

The field of behavioral genetics aims at understanding the observable differences in a wide variety of human characteristics, typically by analyzing the contributions made by heredity and environment in the development of the characteristics in question. Although the research in behavioral genetics is ideologically and methodologically diverse, it is fair to state that it often helps one theorize how much heredity and environment contribute to an observed outcome, and how various factors may interact with each other to create a particular outcome. At the root of such research endeavors lies what is called the nature-nurture controversy.

The Nature-Nurture Controversy

What are the roles of heredity and environment in the development of various human characteristics? The nature-nurture controversy deals with this perennial question. The works by several early philosophers are often viewed as marking the beginning of this controversy. As early as the seventeenth and the eighteenth centuries, philosophers such as René Descartes and Immanuel Kant argued that human cognition is largely reflective of genetically determined predispositions, since environmental factors do not adequately explain the variations in our cognitive capabilities. They therefore took the nativist perspective that humans are born with certain cognitive tendencies. By contrast, the clean slate view, proposed in 1690 by the British philosopher John Locke, focuses instead on the role of the surrounding environment in describing human thoughts. Locke compared the human mind to a piece of blank paper without any ideas written on it, and he suggested that only from experience do humans draw reason and knowledge. Following these diametrically opposed ideas, scientists have since extensively explored the roles of heredity and environment. Before describing such efforts in detail, it is useful to define relevant concepts.

Nature and Nurture Defined

Nature refers to heredity: the genetic makeup or "genotypes" (i.e., information encoded in DNA) an individual carries from the time of conception to the time of death. Heredity may range from genetic predispositions that are specific to each individual and that therefore potentially explain differences in individual characteristics (e.g., temperament), to those supposedly specific to certain groups and that therefore account for group differences in related characteristics (e.g., gender and height), and to those that are theorized to be shared by all humans and are generally thought to set humans apart from other species (e.g., the language acquisition device in humans).

The notion of nature, therefore, refers to the biologically prescribed tendencies and capabilities individuals possess, which may unfold themselves throughout the course of life.

Nurture, by contrast, refers to various external or environmental factors to which an individual is exposed from conception to death. These environmental factors involve several dimensions. For example, they include both physical environments (e.g., secondhand smoking and prenatal nutrition) and social environments (e.g., the media and peer pressure). Also, environmental factors vary in their immediacy to the individual; they involve multiple layers of forces, ranging from most immediate (e.g., families, friends, and neighborhoods) to larger contexts (e.g., school systems and local governments) to macro factors (e.g., international politics and global warming). To complicate matters even further, the factors in each of these layers influence and are influenced by elements within and outside of these layers. For example, the kind of peers a child is exposed to may depend on his or her parents' view of what ideal playmates are like, the local government's housing policies, and the history of race relations.

What Is the Controversy?

Despite its nomenclature, the nature-nurture controversy in its current state is less dichotomous than commonly believed. In other words, the term ''nature-nurture controversy'' suggests a polarization of nature and nurture; continuity and interaction, however, more aptly describe the central processes involved in this controversy. Therefore, it is not about whether either heredity or environment is solely responsible for observed outcomes. Rather, it is more about the extent to which these factors influence human development and the ways in which various factors influence each other.

For example, following the fifteen-person massacre committed by two boys at Columbine High School in Colorado in April 1999, the media were flooded with people offering their interpretations of what drove these high school students to commit this heinous and violent act. Some were quick to attribute the boys' actions to such environmental factors as inadequate parenting practices in their families and the violence prevalent and even glorified in the American media. Others, by contrast, were convinced that these boys were mentally ill as defined in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders and that their ability to make responsible judgments had been impaired, perhaps due to a chemical imbalance to which they were genetically predisposed. Which argument is ''correct,'' according to most researchers? Probably neither. Most theorists agree that both nature and nurture are in tertwined and influence most aspects of human emotion, behavior, and cognition in some ways. Given the prevailing views in current psychology, most researchers would agree that the violent acts committed by these boys probably stemmed from an unfortunate interaction among various hereditary and environmental factors. Researchers, however, may disagree on (1) the extent to which heredity and environment each influences particular developmental outcomes and (2) the way in which a mixture of hereditary and environmental factors relate to each other. In other words, the controversy involves the extent of contribution as well as the nature of interaction among a variety of genetic and environmental forces. How do researchers address these issues?

Exploring Heredity and Environment: Research Methods

Since as early as the 1930s, researchers have attempted to estimate the contribution of hereditary and environmental factors to various aspects of human cognition, by comparing pairs of individuals varying in genetic relatedness. These studies are often called kinship studies, and twin studies and adoption studies represent two of the most common types of such studies. They have been extensively conducted to estimate the heritability of a wide variety of human characteristics.

Twin Studies

In traditional twin studies, monozygotic (identical) twins and dizygotic (fraternal) twins are compared in terms of their emotional, behavioral, and cognitive similarities. In the process of cell divisions upon formation of a zygote, sometimes the resulting cells fully multiply and produce two identical babies; they are called monozygotic twins, since they come from a single zygote and are genetic ''carbon copies.'' In other words, any genetic information concerning physical and psychological predispositions should be exactly the same for these twins.

By contrast, dizygotic twins develop from two separate zygotes, as a result of two eggs being fertilized by two sperms independently. Consequently, the genetic profiles of the resultant babies are similar only to the extent that they share the same set of biological parents. By comparing the correlations of a particular dimension, such as intelligence test scores, between identical twins and those between fraternal twins, researchers can theoretically compute the relative influences of nature and nurture on the dimension. For example, Sandra Scarr reported an interesting finding in the book Intelligence, Heredity, and Environment. She found a correlation for IQ test scores of .86 for identical twins and .55 for fraternal twins, indicating

These children—slumdwellers in 18th-century London—faced few positive prospects as they grew to adulthood. Scientists and philosophers have long struggled with the debate as to how much environmental conditions, both positive and negative, influence individual development. (Archive/Hulton Getty Picture Library)

These children—slumdwellers in 18th-century London—faced few positive prospects as they grew to adulthood. Scientists and philosophers have long struggled with the debate as to how much environmental conditions, both positive and negative, influence individual development. (Archive/Hulton Getty Picture Library)

that identical twins' scores are more like one another than those of fraternal twins. Some influence of heredity, therefore, is evident. If IQ scores were 100 percent genetically determined, however, the correlation for identical twins would have been 1.00. In this example, therefore, heredity appears to play an important, but not definitive, role in explaining the determinants of what is measured through IQ tests.

In addition to these heritability estimates, researchers also study concordance rates: the rates at which both twins develop the same, specific characteristics. The absence or presence of a particular mental illness would be a good example. If both twins had clinical depression in all pairs examined in a study, then the concordance rate would be 100 percent for this sample. On the other hand, if all twins in a study had one individual with clinical depression and another with no depression, then the concordance rate is 0 percent. Reportedly, concordance rate for clinical depression is reportedly about 70 percent for identical twins and about 25 percent for fraternal twins. This appears to demonstrate a sizable genetic contribution involved in the development of depression.

Despite scholars' consensus that genetic contributions are not to be ignored, these correlational data are often believed to be exaggerated. Identical twins are genetically predisposed to a great deal of similarities, and, through a process known as reactive correlation, people around them tend to treat them similarly, which may help lead the twins to be similar beyond what their genetic profiles may warrant. The correlation of .86 between the IQ scores of identical twins, for example, may be contaminated with this reactive correlation. Identical twins encounter environmental experiences that are extremely similar to each other's, as the environment tends to react similarly to those who are genetically similar. As a result, for instance, adults and peers may treat identical twins similarly, and teachers may also develop similar expectations about these twins in terms of their emotional, behavioral, and cognitive functions. This similarity in environmental influences and expectations, therefore, may cause heritability estimates and concordance rates to be exaggerated.

Furthermore, the process of active correlation (or niche-picking) suggests the possibility that children's genetic predispositions cause them to seek particular environments, causing the differences in hereditary predispositions to be enhanced by the subsequent environmental exposure. If a child has the genetic predisposition to enjoy cognitive challenges, for example, that may prompt the child to seek situations, friends, and activities that suit this particular predisposition—provided that such choices are offered to the child. This child, therefore, may start out with a small genetically prompted inclination to want to use his or her ''brains,'' but such a tendency would subsequently be magnified through environmental influences.

Given the varying degrees of genetic similarities between identical and fraternal twins, these sources of confusion may theoretically become more consequential when twins grow up in the same family. This is because twins reared in the same family are typically subject to the same resources, parenting philosophy, living environments, and so on. Their genetic predispositions, therefore, are most likely promoted—or in-hibited—in similar ways. For example, if a pair of twins share the hereditary predispositions for musi-cality and their upper-middle-class parents own a piano and are interested in fostering musicality in these children, their musical potential will perhaps be cultivated in very similar ways. Specifically, their parents will probably get the same or similar piano te-acher(s) for them, and they will probably be encouraged to practice equally. Therefore, the genetic similarities between the twins are magnified by virtue of them growing up in the same household. How does one address these concerns? Adoption studies provide some answers.

Adoption Studies

Compared to traditional twin studies, adoption studies are theorized to offer better alternatives for separating hereditary influences from genetic ones. There are typically two variations in adoption studies: ones involving comparisons of identical twins reared apart and ones comparing the degree of similarity between adopted children and their biological and adoptive parents. Identical twins reared apart share genetic patterns with each other, yet they do not share the same environmental experiences. Adopted children, by contrast, typically share with the rest of the adoptive family similar environmental experiences but do not share any genes with them. The advantage of adoption studies is that researchers can reasonably estimate the heritability by comparing the herit-ability estimates and concordance rates of pairs of individuals varying in genetic relatedness and in environmental distance. A typical adoption study may involve, for instance, comparing the concordance rates for the following two pairs: a child and her biological parent (shared genes but not environments) versus the same child and her adoptive parents (shared environments but not genes). Though the estimates of hereditary influences are generally lower in adoption studies than in twin studies, adoption studies provide results that are largely consistent with twin studies. In a 1983 study, Sandra Scarr and Richard Weinberg found that the IQ scores of adopted children showed higher correlations with the IQ scores of their biological parents than with those of their adopted parents. Similarly, John Loehlin, Lee Willlerman, and Joseph Horn demonstrated through a 1988 study that in the area of clinical depression, adopted children tended to have much higher concordance rates with their biological relatives than with their adoptive relatives.

Still, many scholars argue that heritability may be overestimated in these studies. First, the reactive and active correlations discussed earlier would occur, to a degree, even if the twins were reared separately, as the twins share all of the hereditary predispositions. Second, one must also examine the possibility that parents may systematically treat their adoptive children differently than they do their biological children, which may explain the less-than-expected resemblance between children and their adoptive parents. Given that biologically related individuals tend to share greater hereditary similarities, it is fair to state that heritability estimates may be thrown off by environmental effects induced by particular genetic predispositions.

Beyond Heritability

As illustrated so far, most psychology researchers are in agreement that heredity and environment both play significant roles in the development of various human traits. Researchers may disagree, however, on the extent to which heredity and environment contribute to the development of a particular dimension, and on how various factors may affect each other to create a certain human characteristic. Neither herita-bility estimates nor concordance rates provide useful information on the latter type of disagreement: how various hereditary and environmental factors interact with each other to result in a particular characteristic. Mental health, education, and applied psychology researchers are especially concerned about optimizing the developmental outcomes among people from all backgrounds. To this end, knowing that there is a .86 heritability estimate for IQ scores among identical twins, for example, is not particularly helpful in terms of establishing ways of maximizing the life choices and opportunities for individuals. In attaining such goals, it is crucial to understand how various factors relate to each other. Naturally, in order to do so, one must first identify which factors are involved in the development of a given trait. Unfortunately, researchers have had very limited success in identifying specific genetic patterns that influence particular psychological and behavioral characteristics.

Nevertheless, this is not to suggest that one should ignore the role of heredity as reflected in heri-tability estimates altogether and focus on optimizing the environmental factors for every child. Heredity, as has been examined, undoubtedly contributes to the development of various human traits. Also, researchers exploring environmental influences have found that contrary to what most theorists expected, environmental factors that are shared by reared-together twins do not appear to be relevant in explaining the development of particular traits. It is therefore unlikely that exposing every child to a ''one size fits all'' environment designed to foster a particular trait, would benefit everyone equally. Some may react favorably to such an environment, while others may not react to it at all; there may be yet others who react negatively to the same environment. The notion of ''range of reaction'' helps us conceptualize the complex relationship between heredity and environment; people with varying genetically influenced predispositions respond differently to environments. As suggested by Douglas Wahlsten in a 1994 article in Canadian Psychology, an identical environment can elicit different reactions in different individuals, due to variations in their genetic predispositions. In a hypothetical scenario, Wahlsten suggested that increasing intellectual stimulation should help increase cognitive performances of some children. Moderate, rather than high, levels of intellectual stimulation may, however, induce optimal cognitive performances in others. By contrast, the same moderate levels of stimulation may actually cause some children to display cognitive performances that are even worse than how they performed in a minimally stimulating environment. In addition, the ''optimal'' or ''minimal'' performance levels may be different for various individuals, depending on their genetic makeup and other factors in their lives. This example illustrates the individual differences in ranges of reaction; there is no ''recipe'' for creating environments that facilitate the development of particular characteristics in everyone. Heredity via environment, rather than heredity versus environment, therefore, may better characterize this perspective.

These views are consistent with the 1990s' backlash against the view that was prevalent in the mid- to late twentieth century among many clinical psychologists, social workers, and educators, who focused solely on environmental factors while discounting the contributions of hereditary factors. Among the theories they advocated were that gay males decidedly come from families with domineering mothers and no prominent masculine figures, that poor academic performances result from lack of intellectual stimulation in early childhood, and that autism stems from poor parenting practices. Not surprisingly, empirical data do not support these theories. Still, people often continue to believe, to some extent, that proper environments can prevent and ''cure'' these nonnormative characteristics, not realizing that heredity may play significant roles in the development of these traits.

Some scholars believe that this ''radical environmentalist'' view found its popularity in the 1950s as a reaction to racist Nazi thinking, which held that some groups of individuals are genetically inferior to others and that the undesirable traits they are perceived to possess cannot be prevented or modified. These assumptions are harmful, as they limit the opportunities for advancement of some people, strictly because of their membership in a stigmatized group. It is nevertheless important to reiterate that individual differences, as opposed to group differences, in genetic predispositions are evident in the development of most emotional, behavioral, and cognitive traits. With this in mind, it is also important to realize that focusing on optimizing environmental influences while ignoring hereditary influences may lead to the neglect of the developmental needs of some individuals, and it may be just as harmful in some cases as focusing exclusively on hereditary influences.

See also: PHENOTYPE

Bibliography

American Psychiatric Association. The Diagnostic and Statistical Manual of Mental Disorders: DSM-IV. Washington, DC: American Psychiatric Association, 1994.

Bronfenbrenner, Urie. The Ecology of Human Development: Experiments by Nature and Design. Cambridge, MA: Harvard University Press, 1979.

Efran, Jay, Mitchell Greene, and Robert Gordon. "Lessons of the New Genetics.'' Family Therapy Networker 22 (1998):26—41.

Locke, John. "Some Thoughts concerning Education.'' In R. H. Quick ed., Locke on Education. Cambridge, Eng.: Cambridge University Press, 1892.

Loehlin, John, Lee Willerman, and Joseph Horn. "Human Behavior Genetics.'' Annual Review of Psychology 38 (1988):101-133.

Lykken, David. The Antisocial Personality. Hillsdale, NJ: Lawrence Erlbaum, 1995.

McGee, Mark, and Thomas Bouchard. "Genetics and Environmental Influences on Human Behavioral Differences.'' Annual Review of Neuroscience 21 (1998):1—24.

McGuffin, Peter, and Michael Pargeant. "Major Affective Disorder.'' In Peter McGuffin and Robin Murray eds., The New Genetics of Mental Illness. London: Butterworth-Heinemann, 1991.

Newman, H. H., F. N. Freeman, and K. J. Holzinger. Twins: A Study of Heredity and Environment. Chicago: University of Chicago Press, 1937.

Plomin, R. Genetics and Experience: The Interplay between Nature and Nurture. Thousand Oaks, CA: Sage, 1994.

Plomin, Robert, J. C. DeFries, and John Loehlin. "Genotype-Environment Interaction and Correlation in the Analysis of Human Behavior.'' Psychological Bulletin 84 (1977):309-322.

Scarr, Sandra. ''Behavior-Genetic and Socialization Theories of Intelligence: Truce and Reconciliation.'' In R. J. Sternberg and E. L. Grigorenko eds., Intelligence, Heredity, and Environment. New York: Cambridge University Press, 1997.

Scarr, Sandra, and Richard Weinberg. "The Minnesota Adoption Studies: Genetic Differences and Malleability." Child Development 54 (1983):260-267.

Waddington, C. H. The Strategy of the Genes. London: Allen and Unwin, 1957.

Wahlstein, Douglas. ''The Intelligence of Heritability.'' Canadian Psychology 35 (1994):244-259.

Daisuke Akiba

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  • benjamin
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