What is behavior? |

Brain Biology

As the first organ system to begin development and the last to be completed, the vertebrate nervous system—brain, spinal cord, and nerves—with the brain at the control, remains something of an enigma to biologists and other scientists. The vertebrate brain comprises, among other structures, neurons, which are special cells that generate and transmit bioelectrical impulses via a number of different neurotransmitters. The brain consists of three major neural structures: the brain stem, the cerebellum, and the cerebrum. A reptilian brain consists of only the brain stem, while the mammalian brain has all three, including a well-developed cerebrum (the two large hemispheres on top). The brain stem controls basic body functions such as breathing and heart rate, while the cerebrum is the ultimate control center. Consisting of billions of neurons (commonly called brain cells), the cerebrum controls such higher-level functions as memory, speech, hearing, vision, and analytical skills.

Scientists have long sought to understand the complex relationship between the brain, behavior, and genetics. Decades of research have led to a general consensus that fundamental to human behavior, cognition, and emotions is the functioning of the cerebral cortex (that is, for higher-brain functions) and the limbic system, which includes the amygdala, septum, cingulate, hippocampus, anterior thalamic nuclei, fornix, and mammillary bodies. In particular, the human brain is governed by the frontal lobes of the cerebral cortex, which controls cognitive processes. And connecting the brain stem and the cortex is the limbic system, the center that mediates motivated behaviors, emotional states, and memory, as well as regulates temperature, blood-sugar levels, and blood pressure.

Clearly, neurotransmitters influence behavior and cognition by modulating the activity of neurons. More specifically, the brain—at all levels—is an exceedingly complex network of billions of neurons. As messages enter the brain stem from the spinal cord, groups of neurons either respond directly or transfer information to higher levels. In order to communicate with other neurons, each individual neuron generates impulses much like the impulse that carries a digital signal over a fiber-optic cable, and this message travels from the beginning to the end of each neuron. At the end of one neuron and the beginning of the next in line, a small open space exists. Here the message is carried across to the next neuron by a chemical known as a neurotransmitter.

Neurotransmitters are of several biochemical classifications, including acetylcholines, amines, amino acids, and peptides. An individual neuron and an entire neuronal circuit may fire or not fire an impulse based on the messages carried by these neurotransmitters. For example, the signal for pain is transmitted from neuron to neuron by a peptide-based neurotransmitter known as substance P, while another peptide transmitter (endorphin) acts as a natural painkiller. Thought, memory, and behavior, then, are produced by the activity along neuronal circuits. A genetic link occurs here, since neurotransmitters are expressed either directly or indirectly based on information in genes.

By birth, the collection of approximately 24,000 genes in humans has guided the development of the nervous system. At birth, the brain consists of approximately 100 billion neurons and trillions of supporting glial cells to protect and nourish neurons. However, the intricate wiring between these neurons, including exactly how a virtual multitude of neural signals eventually translates into thinking and behavior, remains to be determined.

While many scientists have held to a strictly genetic model in order to explain human behavior and cognitive functioning, others have suggested that the critical networking and circuit formation between these billions of neurons that control later brain function are determined not from genes but from environmental input and experiences from birth until the brain is fully developed around age seven. In other words, controversy has arisen as scientists have attempted to explain behavior from a genetic, inherited perspective versus from a social and environmental one. Typically the nature-versus-nurture debate, which has been ongoing for centuries, has been a dichotomy: either nature or nurture. What has developed is a sense that both play a role, with the controversy centering around to what degree either nature or nurture predominates as the primary causal factor in any given trait outcome.

Just how many human traits and abilities are innate or acquired through interactions with the environment is unknown, however. On one end of the continuum is John Locke’s concept of the tabula rasa, in which the human brain of the newborn is thought to be a “blank slate” that will be differentiated only by sensory experiences. On the other end is today’s biological determinism, in which behavior is thought to be strictly innate and heritable. A majority of experts subscribe to a middle-of-the-road view that rests between these two extremes of this nature-versus-nurture argument. For example, a spider phobia might be considered to result from a combination of an innate evolution-driven fear of potentially dangerous spiders, a heritable tendency toward anxiety, and conditioning through prior “bad” experiences with insects.

Genes and Behavior

Traditionally, the field of behavioral genetics has emphasized evaluating how much population variance is determined by environmental or hereditary factors. From the perspective of human development and behavior, the issue also becomes one of how this process is expressed within cultural constraints—by what means genetic and social surroundings reciprocate to yield obvious outcomes.

Genes make proteins, and proteins cause biochemical responses in cells. The behavior of an animal takes place under the combined influences of its genes, expressed through the actions of proteins, and its environment. A good example is the phenomenon of mating seasons in many animals. As day length gradually increases toward spring and summer, a critical length is reached that signals the release of hormones that result in increased sexual activity, with the ultimate goal of seasonal mating. The production and activity of hormones involve genes or gene products. If the critical number of daylight hours is not reached, the genes will not be activated, and sexual behavior will not increase.

Each neuron making up the intricate networks and circuits throughout the cerebrum (about 80 percent of the human brain) has protein receptors (chemoreceptors) that respond to specific signaling molecules. The production of the receptors and signaling molecules used for any type of brain activity is directly tied to genes. A slightly different gene may lead to a slightly different signaling molecule or receptor and thus a slightly different cell (neuron) response. A larger difference among genes may lead to a larger difference among signaling molecules or receptors and thus a larger variation in cell response. Since human behavior involves the response of neurons and neuron networks in the brain to specific signals, and because the response of neurons occurs from the interaction between a signaler and a receptor built by specific genes, the genetic link seems straightforward: input, signal, response, behavior. However, when the slight variations between genes are added to the considerable variation among noncoding or regulatory sequences of DNA, the genetic connection to behavior becomes much less direct. Because a gene is under the control of one or several regulatory sequences that in turn may be under the control of various environmental inputs, the amount of genetic variation among individuals is compounded by two other critical factors: the environmental variations under which the brain develops and the daily environmental variations to which the individual is exposed. A convenient way to think of genetics and behavior is to consider that genes allow humans to respond to a specific stimulus by building the pathway required for a response, while behavior is defined by the degree and the manner of human response.

Eugenics

Eugenics is the categorization of a specific human behavior to an underlying genetic cause. People inherit specific genes to build specific pathways that allow them to respond in certain ways to environmental input. With variations possible—from the gene-to-gene regulators to the final cellular response—it is virtually impossible to disconnect the nature-versus-nurture tie that ultimately controls human behavior. Genes are simply the tools by which the environment shapes and reshapes human behavior. There is a direct correlation between gene and protein: Change the gene, change the protein. However, there is no direct correlation between gene and behavior: Changing the gene does not necessarily change the behavior. Behavior is a multifaceted, complex response to environmental influences that is only partially related to genetic makeup.

Another important fact is that almost no behaviors are controlled by a single gene locus, and the more complex the behavior, the more likely that it is controlled by several to many genes. Hence, not only do environmental effects cloud the picture, but each gene involved in more complex behavioral traits represents just a small part of the genetic basis for the trait as well. The study of the genetic basis for complex traits, therefore, involves the search for quantitative trait loci (QTLs), rather than for single genes.

Searching for QTLs requires that a large number of genetic markers be identified in the human genome, and the Human Genome Project has provided numerous such markers. A QTL is identified by looking for “linkage” between a specific genetic marker and the trait being studied. Linkage occurs when a marker is close to one of the genes that control the trait. Practically speaking, this means that individuals with the behavioral trait have the marker, and those who do not have the trait lack the marker. Thus, geneticists are not directly identifying the genes involved, but are identifying the approximate locations of the genes. Unfortunately, the more genes that control a trait, the harder it is to identify QTLs. Environmental effects can also mask the existence of QTLs, causing some people to have the trait that lack a QTL and others to lack the trait but have a QTL. In spite of these difficulties, QTLs have been identified for
a number of behavioral traits, such as aggression, depression, and a number of other mental disorders.

Single-Gene Behavioral Traits

Although behavioral traits controlled by a single gene have been identified, they probably require interaction with other genes in order to produce the specific characteristics of the behavior. On top of this are laid environmental effects. The most dramatic case of a single gene that controls a complex behavior was the discovery in the early twenty-first century of a gene that controls honeybee social status. This same gene is found in fruit flies and affects how actively fruit flies seek food. Bees with a more actively expressed form of the gene (called the for gene) were much more likely to forage than bees with a less active for gene. Not surprisingly, the for gene produces a protein that acts as a cell-signaling molecule.

In humans, only a few behavioral traits are clearly controlled by a single gene. The best examples are Huntington’s disease
(a rare, autosomal dominant gene), early-onset Alzheimer’s disease (also a rare, autosomal dominant gene), and fragile X syndrome
(actually involves two genes). The remaining traits, as far as has been determined to date, probably represent multigene traits where one primary QTL has been identified as primarily responsible.

Several genes were identified, beginning in the late 1980s, with possible direct behavioral links. Genes have been implicated in such behaviors as anxiety, depression, hostility, and impulsiveness. One such gene produces a protein that transports a chemical called serotonin, across neuronal membranes. Serotonin is a neurotransmitter and is the chemical that is affected by the antidepressant drug Prozac and other selective serotonin reuptake inhibitors (SSRIs). Scientists have also identified a gene that may be related to schizophrenia
and a gene that may determine how well alcohol is cleared from the brain after overindulgence.

One of the more recent, and in some ways controversial, discoveries involved a gene for antisocial behavior (ASB). The study followed the lives of more than one thousand boys from birth. Children who grew up in abusive environments were more likely to display antisocial behavior later, which is not a surprise. However, about half of the boys were found to have lower levels of an enzyme called monoamine oxidase A (MAOA), which is involved in the metabolism of several neurotransmitters. The boys with the lower MAOA activity were twice as likely to have been diagnosed with conduct disorder and were three times more likely to have been convicted of a violent crime by age twenty-six. It should be noted that lower MAOA activity alone was not enough; the
boys also had to be exposed to abusive upbringings. Although the link seems strong, it has not been proved, with continued study needed.

In short, a more thorough understanding of single-gene behavioral traits could open the way to more accurate diagnoses and better treatments.

Multiple-Gene Behavioral Traits

Geneticists concede that for many behavioral traits it may never be possible to sort out the details of the underlying genetic causes. Still, theories abound and researchers continue to speculate. Some genes may play such a minor role that the search for some QTLs will be fruitless. Nevertheless, geneticists have been able to discover QTLs for some important behavioral traits, and the heritability of a number of traits has been determined. The better data available from the Human Genome Project have spawned the rapidly growing field of behavioral genomics, with its emphasis on identifying the specific genetic mechanisms involved in the determination of behavior.

Nonetheless, the quality of the environment matters in most cases. A practical example of this is intelligence or IQ, which is thought by many experts to involve both environmental and genetic influences, given individual abilities to adapt to social stressors. Successful adaptation requires personal coping but may also require either altering the quality of the present surroundings or locating another environment altogether. Such intentional coping also requires a number of mental processes, including sensation, perception, memory, reasoning, learning, and problem solving. The primary thrust is to avoid labeling human mental functioning as strictly nature or nurture, but rather as a selective combination of multiple adaptive processes employed for successful coping in the environment. In short, certain traits may never be fully understood from a strictly genetic perspective. Even when heritability is high, the environment also plays an important role, and numerous genes are likely involved.

More success has come from focusing on specific disorders. For example, a series of genes have been identified that may be involved in attention deficit hyperactivity disorder. Other QTLs have been identified in some studies but have not been found in others. This shows one of the frustrating aspects of studying the genetics of behavior. QTLs identified using one set of data will not be supported by another set of data. This may be true because such QTLs play such a small part in developing the trait that they are undetectable under certain environmental conditions. Genes and QTLs for dyslexia and schizophrenia have also been discovered.

For the most complex human traits, QTLs still await discovery, but twin studies have perhaps yielded the most valuable data regarding the relationship between genes and behavior. Twin studies involve comparing the traits of identical twins that were separated from birth. The assumption is that, because they have been raised in different environments, any traits they share will be primarily attributable to genetics rather than to environment. An early study of Swedish men showed that heritability of cognitive (thinking) ability was 62 percent, while spatial ability was 32 percent. Heritability of other personality traits fell somewhere between these values. Although these kinds of studies are interesting, they can be misleading unless considered in proper context. Consequently, a number of geneticists criticize such research, especially twin studies, as having some inherent statistical problems. These studies can also lead to misunderstandings, especially by nonscientists, who often interpret the
numbers incorrectly. For example, saying that cognitive ability has a 62 percent heritability does not mean that a child has a 62 percent chance of being as intelligent as his or her parents but rather that, of the factors involved in determining a person’s intelligence, genetics accounts for approximately 62 percent of the observed variation in the population.

The Future of Behavioral Genetics

Researchers continue to actively investigate the potential links between behavior and genetics in human functioning. Even when such links are found, however, the degree to which a particular gene is involved and the amount of variation among humans will likely be hard to uncover. The Human Genome Project has greatly accelerated interest in and the search for the genetic bases of behavior, yet with these new data have come an even clearer realization of the complexities of the interplay between genes and behavior. If nothing else, the future should hold more precise answers to the long-standing questions about what makes human beings who they are. The consensus among geneticists today is that behavior is determined neither solely by genes nor solely by the environment. To this end, further research should attempt to make the relative contributions of genes and environment more understandable.

Key Terms



eugenics

:

a process in which negative genetic traits are removed from the population and positive genetic traits are encouraged, by controlling, in some manner, who is allowed to reproduce




genome

:

the entire set of genes required by an organism; a set of chromosomes




heritability

:

the probability that a specific gene or trait will be passed from parent to offspring, rendered as a number between 0 and 100 percent, with 0 percent being not heritable and 100 percent being completely heritable




Human Genome Project (HGP)

:

an international genetics project developed to identify and map the human genome with its approximate 24,000 genes, the first assembly of which was completed by the UCSC Genome Bioinformatics Group in 2003




linkage

:

a relation of gene loci on the same chromosome; the more closely linked two loci are, the more often the specific traits controlled by these loci are expressed together




neurotransmitter

:

a chemical messenger that transmits a neural impulse between neurons




population genetics

:

the discipline within the field of evolutionary biology concerned with the study of changes in gene frequency, including how this relates to human groups





Bibliography

Avital, Eytan, and Eva Jablonka. Animal Traditions: Behavioural Inheritance in Evolution. New York: Cambridge UP, 2000. Print.

Badcock, C. R. Evolutionary Psychology: A Critical Introduction. Malden: Polity Press, 2000. Print.

Benjamin, Jonathan, Richard P. Ebstein, and Robert H. Belmaker, eds. Molecular Genetics and the Human Personality. Washington, DC: APA, 2002. Print.

Briley, Mike, and Fridolin Sulser, eds. Molecular Genetics of Mental Disorders: The Place of Molecular Genetics in Basic Mechanisms and Clinical Applications in Mental Disorders. Malden: Blackwell, 2001. Print.

Burnham, Terry, and Jay Phelan. Mean Genes: From Sex to Money to Food—Taming Our Primal Instincts. Cambridge: Perseus, 2000. Print.

Carson, Ronald A., and Mark A. Rothstein. Behavioral Genetics: The Clash of Culture and Biology. Baltimore: Johns Hopkins UP, 1999. Print.

Cartwright, John. Evolution and Human Behavior: Darwinian Perspectives on Human Nature. Cambridge: MIT Press, 2000. Print.

Cicchetti, Dante, et al. "Behavior Genetics: Past, Present, Future." Development & Psychopathology 25.4.2 (2013): 1225–42. Print.

Clark, William R., and Michael Grunstein. Are We Hardwired? The Role of Genes in Human Behavior. New York: Oxford UP, 2000. Print.

DeMoss, Robert T. Brain Waves Through Time: Twelve Principles for Understanding the Evolution of the Human Brain and Man’s Behavior. New York: Plenum Trade, 1999. Print.

Kim, Yong-Kyu. Handbook of Behavior Genetics. New York: Springer, 2009. Print.

Longino, Helen E. Studying Human Behavior: How Scientists Investigate Aggression and Sexuality. Chicago: U of Chicago P, 2013. Print.

Plomin, Robert, et al. Behavioral Genetics. 6th ed. New York: Worth, 2013. Print.

Rosen, David H., and Michael C. Luebbert, eds. Evolution of the Psyche. Westport: Praeger, 1999. Print.

Rutter, Michael. Genes and Behavior: Nature-Nurture Interplay Explained. New York: Wiley-Blackwell, 2006. Print.

Wright, William. Born That Way: Genes, Behavior, Personality. New York: Knopf, 1998. Print.