Judicature Genes and Justice
The Growing Impact of the New Genetics on the Courts
November-December 1999 Vol 83(3)
GENES AND BEHAVIOR
A complex relationship
by Joseph D. McInerney
Sometimes lessons in biology some unexpectedly, as when a rottweiler appeared over my right shoulder while I was kneeling to trim some rose bushes early this summer. He was large—more so for being at eye level—and he was wearing a collar made of chain links large enough to anchor a small ship. I turned to face him as an archive of rottweiler news stories came up from long-term memory, none of them happy, all of them populated by mutilated people and by dogs destroyed at the direction of the authorities.
What to do next? How did I appear to this animal? Did I look like a threat? Did I look like lunch? He certainly wasn't behaving in a menacing manner, but maybe he was waiting for the right moment to express his breed's well-known, vicious disposition. One of us had to do something, I figured, although reason, the pride of Homo sapiens, was little comfort in the face of the evolutionary legacy of Canis familiaris: powerful jaws and teeth adapted to gripping and tearing. I extended my hand slowly, and—he licked it. The dog worked his way up to my face with a tongue as broad and soggy as a kitchen sponge, and, as his owner appeared, I expressed my relief at seeing such friendly behavior from a representative of so notorious a breed. The young man replied, "These dogs get a bad rap. People say they're born mean. Look at Caesar, here. Ain't no dog born mean. You got to teach them to be mean."
In their different ways, Caesar and his human friend raised long-standing questions about the roots of animal behavior, including behavior in our own species. Are behaviors inbred, written indelibly in our genes as immutable biological imperatives, or is the environment more important in shaping our thoughts and actions? Such questions cycle through society repeatedly, forming the public nexus of the "nature vs. nurture controversy," a strange locution to biologists, who recognize that behaviors exist only in the context of environmental influence. Nonetheless, the debate flares anew every few years, reigniting in response to genetic analyses of traits such as intelligence, criminality, or homosexuality, characteristics freighted with social, political, and legal meaning.
Sir Francis Galton (1822-1911) was the first scientist to study heredity and human behavior systematically. He focused on behavioral correlations within families and developed a few research techniques still in use today—twin studies, for example. Galton also arrived at some interesting conclusions, including this 1907 summary of the inheritance of criminal tendencies: The ideal criminal has marked peculiarities of character: his conscience is almost deficient, his instincts are vicious, his power of self-control is very weak, and he usually detests continuous labor. The absence of self-control is due to ungovernable temper, to passion, or to mere imbecility, and the conditions that determine the particular descriptions of crime are the character of the instincts and of the temptation.
The perpetuation of the criminal class by heredity is a question difficult to grapple with on many accounts.... It is, however, easy to show that the criminal nature tends to be inherited.... The true state of the case appears to be that the criminal population receives steady accessions from those who, without having strongly marked criminal natures, do nevertheless belong to a type of humanity that is exceedingly ill suited to play a respectable part in our modern civilization, though it is well suited to flourish under half-savage conditions, being naturally both healthy and prolific. [Galton, Inquiry into Human Faculty and its Development, 2nd edition (London: J.M. Dent & Sons, Ltd., 1907).]
This passage demonstrates Galton's conviction about the hereditary basis of criminal behavior, by no means established even now, but he wrote elsewhere in the same volume about the "difficulty of distinguishing that part of (man's) character which has been acquired through education and circumstance, and that which was in the original grain of his constitution." The difficulty persists, notwithstanding an explosion of data about human genes and the development of molecular and statistical tools that Galton could not have imagined.
The term "genetics" did not even appear until 1909, only two years before Galton's death, but with or without a formal name, the study of heredity always has been, at its core, the study of biological variation. Human behavioral genetics, a relatively new field, seeks to understand both the genetic and environmental contributions to individual variations in human behavior. That is not an easy task, for the following reasons.
Research in behavioral genetics proceeds from the assumption that behavior is rooted in biology, a conclusion shared by all biologists and obvious even to the nonspecialist after a few moments of reflection (see "Indications that behavior has a biological basis"). Simply accepting that conclusion, however, is a lot easier than elucidating underlying biological mechanisms. Furthermore, even casual observation tells us that biology is not the whole story. If it were, there would not be discrepant phenomena such as schizophrenics whose identical twins—naturally occurring clones—are unaffected by the disease, or Caesar, the antithesis of his bad-tempered rottweiler cousins.
- It often is difficult to define the behavior in question. Intelligence is a classic example. Is intelligence the ability to solve a certain type of problem? The ability to make one's way successfully in the world? The ability to score well on an IQ test? During the late summer of 1999, a Princeton molecular biologist published the results of impressive research in which he enhanced the ability to learn in mice by inserting a gene that codes for a protein in brain cells known to be associated with memory. Because the experimental animals performed better than controls on a series of traditional tests of learning, the press dubbed this gene "the smart gene" and the "I.Q. gene," as if improved memory were the central, or even sole, criterion for defining intelligence. In reality, there is no universal agreement on the definition of intelligence, even among those who study it for a living.
- Having established a definition for research purposes, the investigator still must measure the behavior with acceptable degrees of validity and reliability. That is especially difficult for basic personality traits, such as shyness or assertiveness, which are the subject of much current research. Sometimes there is an interesting conflation of definition and measurement, as in the case of IQ tests, where the test score itself has come to define the trait it measures. This is a bit like using batting average to define hitting prowess in baseball. A high average may indicate ability, but it does not define the essence of the trait.
- Behaviors, like all complex traits, involve multiple genes, a reality that complicates the search for genetic contributions.
- As with much other research in genetics, studies of genes and behavior require analysis of families and populations for comparison of those who have the trait in question with those who do not. The result often is a statement of "heritability," a statistical construct that estimates the amount of variation in a population that is attributable to genetic factors. The explanatory power of heritability figures is limited, however, applying only to the population studied and only to the environment in place at the time the study was conducted. If the population or the environment changes, the heritability most likely will change as well. Most important, heritability statements provide no basis for predictions about the expression of the trait in question in any given individual.
Traditional research strategies in behavioral genetics include studies of twins and adoptees, techniques designed to sort biological from environmental influences. More recently, investigators have added the search for pieces of DNA associated with particular behaviors, an approach that has been most productive to date in identifying potential locations for genes associated with major mental illnesses such as schizophrenia and bipolar disorder. Yet even here there have been no major breakthroughs, no clearly identified genes that geneticists can tie to disease. The search for genes associated with characteristics such as sexual preference and basic personality traits has been even more frustrating.
Such are the allure and misunderstanding of genetics among press and public, however, that even preliminary findings of genetic influence provoke misleading statements about "genes for" a particular behavior, as if genetic causation had been established. In fact, genes can do nothing by themselves. All of their actions and influence are mediated by proteins—gene products—and until we understand something about the proteins involved in the myriad steps that produce a given trait, and about the individual uniqueness to which they contribute, it is difficult to propose a plausible biological explanation for the trait's expression. The uniqueness is compounded by the non-linear nature of those myriad steps. Indeed, pervasive uniqueness suggests that there is no fixed essence in human behavior, only variation, a concept central to all of biology and one that Galton's famous cousin, Charles Darwin, used to build his revolutionary theory of evolution by natural selection.
To this already complex calculus we must add the knowledge that biological processes that combine to produce behaviors or any other complex traits cannot exist apart from the unique experiences of the individual, perhaps dating as far back as experiences in the womb. An accounting of those experiences and of their interactions with one's unique biological constitution would confound our ability to make sound predictions about the occurrence of a given behavior, even if we knew that predisposing genes were present.
Genetics and molecular biology have provided some significant insights into behaviors associated with inherited disorders. For example, we know that an extra chromosome 21 is associated with the mental retardation that accompanies Down's syndrome, although the processes that disrupt brain function are not yet clear. We also know the steps from gene to effect for a number of single-gene disorders that result in mental retardation, including, phenylketonuria (PKU), a treatable metabolic disorder for which all newborns in the United States are tested.
In general, it is easier to discern the relationship between biology and behavior for chromosomal and single-gene disorders than for common, complex behaviors that are of considerable interest to specialist and nonspecialist alike. So the former are at the more informative end of a sliding scale of certainty with respect to our understanding of human behavior. At the other end of the scale are the hard-to-define personality traits, while somewhere in between are traits such as schizophrenia and bipolar disorder—organic diseases whose biological roots are undeniable yet unknown, and whose unpredictable onset teaches us about the importance of environmental contributions even as it reminds us of our ignorance.
The Human Genome Project doubtless will provide researchers with the data they need to identify individual genes or suites of genes that contribute to human behaviors. The really hard work only begins at that point, however, with analysis of the ways in which the products of those genes influence human growth and development, of the environmental influences on those processes, and of the degree of individuality of both. So, the likelihood that we soon will use genetic analysis to predict the behavior of a given person or to explain a behavior already expressed—a criminally violent act, for example—is not great. Those who study genes and behavior, however, are confident about one thing: The debate about nature vs. nurture is empty; the prevailing view is one of how nature and nurture contribute to the individuality of behavior.
Joseph D. McInerney is director of the Foundation for Genetic Education and Counseling. He is grateful to Barton Childs, M.D., Johns Hopkins Hospital, for a helpful review of this article.
|The online presentation of this publication is a special feature of the Human Genome Project Information Web site.|