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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
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
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.
How do genes influence behavior?
No single gene determines a particular behavior. Behaviors are complex
traits involving multiple genes that are affected by a variety of other
factors. This fact often gets overlooked in media reports hyping scientific
breakthroughs on gene function, and, unfortunately, this can be very misleading
to the public.
For example, a study published in 1999 claimed that overexpression of a particular
gene in mice led to enhanced learning capacity. The popular press referred to
this gene as "the learning gene" or the "smart gene." What
the press didn't mention was that the learning enhancements observed in this
study were short-term, lasting only a few hours to a few days in some cases.
Dubbing a gene as a "smart gene" gives the public a false impression
of how much scientists really know about the genetics of a complex trait
like intelligence. Once news of the "smart gene" reaches the
public, suddenly there is talk about designer babies and the potential
of genetically engineering embryos to have intelligence and other desirable
traits, when in reality the path from genes to proteins to development
of a particular trait is still a mystery.
With disorders, behaviors, or any physical trait, genes are just a part
of the story, because a variety of genetic and environmental factors are
involved in the development of any trait. Having a genetic variant doesn't
necessarily mean that a particular trait will develop. The presence of
certain genetic factors can enhance or repress other genetic factors.
Genes are turned on and off, and other factors may be keeping a gene from
being turned "on." In addition, the protein encoded by a gene
can be modified in ways that can affect its ability to carry out its normal
Genetic factors also can influence the role of certain environmental
factors in the development of a particular trait. For example, a person
may have a genetic variant that is know to increase his or her risk for
developing emphysema from smoking, an environmental factor. If that person
never smokes, then emphysema will not develop.
Where can I learn more about the genetics
of different behavioral traits?
Online Mendelian Inheritance in Man (OMIM) is a large, searchable, up-to-date
database of human genes, genetic traits, and disorders. Each OMIM record
contains bibliographic references and a summary of the scientific literature
describing what is known about a particular gene, trait, or disorder.
The following behavioral traits are included in OMIM. The six-digit number
MIM number is used to uniquely identify each record.
- Hand skill, relative (handedness): (139900)
- Hand clasping pattern: (139800)
- Arm folding preference: (107850)
- Ears, ability to move: (129100)
- Tongue curling, folding, or rolling: (189300)
- Musical perfect pitch: (159300)
- Novelty seeking personality trait: (601696)
- Stuttering: (184450)
- Tobacco addiction: (188890)
- Alcoholism: (103780)
- Homosexuality: (306995)
You also may want to search OMIM for behavioral traits not included in
the list above. For step-by-step instructions, see our OMIM
Search Tutorial. For more detailed information, review the Help
pages. For information on other databases of human genes, see the Gene
and Protein Database Guide available through Gene
Behavioral Genetics Links
- University of Pennsylvania
Behavioral Genetics Laboratory
- Virginia Institute for Psychiatric
and Behavioral Genetics
- Behavioral Genetics in the Postgenomic Era, by Robert Plomin,
John C. Defries, Ian Craig, and Peter McGuffin, eds., and Jerome Kagan.
2002, 608 pp.
Genetics: The Clash of Culture and Biology by
Ronald A. Carson and Mark A. Rothstein. 1999, 224 pp.
- Behavioral Genetics by Robert Plomin (Editor),
John C. Defries, Gerald E. McClearn, Peter McGuffin. 2000, 4th edition,
- Living With Our Genes: Why They Matter More Than You Think
by Dean H. Hamer and Peter Copeland. 1999, 368 pp.
- Genetics of Mental Disorders: A Guide for Students, Clinicians,
and Researchers by S.V. Faraone, M.T. Tsuang, and
D.W. Tsuang. Guilford Press (1999), 272 pp.