Download What is behavioral genetics?

What is behavioral genetics?
[text provided by Joseph McInerney]
Sir Francis Galton (1822-1911) was the first scientist to study heredity and human behavior
systematically. The term "genetics" did not even appear until 1909, only 2 years before Galton's
death. 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. This is not an easy task, for the following reasons.

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 of mice to learn 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 "IQ 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 averages 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.
What indications are there that behavior has a biological basis?
[text provided by Joseph McInerney]

Behavior often is species specific. A chickadee, for example, carries one sunflower seed
at a time from a feeder to a nearby branch, secures the seed to the branch between its feet,
pecks it open, eats the contents, and repeats the process. Finches, in contrast, stay at the
feeder for long periods, opening large numbers of seeds with their thick beaks. Some
mating behaviors also are species specific. Prairie chickens, native to the upper Midwest,
conduct an elaborate mating ritual, a sort of line dance for birds, with spread wings and
synchronized group movements. Some behaviors are so characteristic that biologists use
them to help differentiate between closely related species.

Behaviors often breed true. We can reproduce behaviors in successive generations of
organisms. Consider the instinctive retrieval behavior of a yellow Labrador or the herding
posture of a border collie.

Behaviors change in response to alterations in biological structures or processes. For
example, a brain injury can turn a polite, mild-mannered person into a foul-mouthed,
aggressive boor, and we routinely modify the behavioral manifestations of mental
illnesses with drugs that alter brain chemistry. More recently, geneticists have created or
extinguished specific mouse behaviors—ranging from nurturing of pups to continuous
circling in a strain called "twirler"— by inserting or disabling specific genes.

In humans, some behaviors run in families. For example, there is a clear familial
aggregation of mental illness.

Behavior has an evolutionary history that persists across related species.
Chimpanzees are our closest relatives, separated from us by a mere 2 percent difference
in DNA sequence. We and they share behaviors that are characteristic of highly social
primates, including nurturing, cooperation, altruism, and even some facial expressions.
Genes are evolutionary glue, binding all of life in a single history that dates back some
3.5 billion years. Conserved behaviors are part of that history, which is written in the
language of nature's universal information molecule—DNA.
How is behavioral genetics studied?
[text provided by Joseph McInerney]
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.
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.
What implications does behavioral genetics research have for society?
[text provided by Joseph McInerney and Mark Rothstein]
Researchers in the field of behavioral genetics have asserted claims for a genetic basis of
numerous physical behaviors, including homosexuality, aggression, impulsivity, and nurturing.
A growing scientific and popular focus on genes and behavior has contributed to a resurgence of
behavioral genetic determinism—the belief that genetics is the major factor in determining
behavior.
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.
What social consequences would genetic diagnoses of such traits as intelligence, criminality, or
homosexuality have on society? What effect would the discovery of a behavioral trait associated
with increased criminal activity have on our legal system? If we find a "gay gene," will it mean
greater or lesser tolerance? Will it lead to proposals that those affected by the "disorder" should
undergo treatment to be "cured" and that measures should be taken to prevent the birth of other
individuals so afflicted?
There are several scientific obstacles to correlating genotype (an individual's genetic
endowment) and behavior. One problem is in defining a specific endpoint that characterizes a
condition, be it schizophrenia or intelligence. Another problem is in identifying and excluding
other possible causes of the condition, thereby permitting a determination of the significance of a
supposed correlation. Much current research on genes and behavior also engenders very strong
feelings because of the potential social and political consequences of accepting these supposed
truths. Thus, more than any other aspect of genetics, discoveries in behavioral genetics should
not be viewed as irrefutable until there has been substantial scientific corroboration.