Heredity Versus Environment

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 intertwined 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?

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) 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 inhibited—in similar ways. For example, if a pair of twins share the hereditary predispositions for musicality 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 teacher(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 heritability 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.