In 1991 Stephen Mobley shot a Georgia pizza parlor employee in the back of the head and bragged about the shooting to his friends. Last year, Mobley appealed his conviction
to the state's Supreme Court. His new defense: my genes made me do it.
Pointing to a long family history of genetic illness and criminal behavior, Mobley's attorneys argue that a range of genetic factors made it almost inevitable that he would eventually come to lead a life of crime. The Mobley case stands at the center of the age-
old "nature versus nurture" debate in the development of human behavior.
Interestingly enough, Mobley may have a good case: some recent studies and new techniques, have given new life to the one hundred and fifty year old quest to find a link
between behavior and genes.
Genes and Traits
Gregor Mendel, the father of modern genetics, found single genes responsible for producing individual traits in his landmark experiments with peas in the 19th century. He noticed that his peas’ phenotypes, or actual appearances, were dictated by their genotypes, or genetic makeup. While Mendel could develop a simple model of inheritance of single traits for the all the peas in his garden, human genetics is complicated by the fact that many traits we possess are dependent upon the actions of many genes as well as environmental factors. The traits Mendel studied in his peas facilitated genetic analysis because they fell into distinct categories such as dwarf or tall, yellow or green. Most human traits cannot be described in such a simple manner, but instead exist as a continuous distribution of characteristics. Height is one such trait.
Human behavior is another, but is even more complex than height because it cannot be quantified as readily. The difficulty in determining a method for measuring behavior makes the problem of defining the relationship between genotypes and behavioral phenotypes extremely challenging.
Genetics, Eugenics, and Nazis
Sir Francis Galton, inspired by his cousin Darwin's Origin of Species (1859), pioneered the field of behavioral genetics, developing a school of thought called hereditarianism.
Galton believed all human traits, including behavior, are solely determined by genes, without any contribution from the environment. This idea became the basis for eugenics, a term Galton coined in 1883 to describe the use of genetics for social planning. Galton, in other words, believed that genotype alone determined most characteristics, and that selective "breeding" of the human species could guarantee that the "best" traits would remain in the human population while the "worst" traits could be eliminated.
Based on Galton's ideas, the eugenics movement built momentum in the United
States, where between 1905 and 1933, several states based laws and legal decisions on eugenics theory, and the federal government heavily restricted immigration from Eastern and Southern Europe. The goal was the production of a genetically "pure" American

population. During this time in American history, criminals, people with low IQs and, in some cases, even women who had illegitimate children, were sterilized.
During its time in power, the government of Nazi Germany staunchly supported and enforced the principles of eugenics. Eugenics-based legislation culminated in the
"euthanasia" of the mentally retarded and the physically disabled, who, according to
Nazi geneticists, would otherwise contaminate the German people with unfit genes. The list of genetic undesirables ultimately grew to include Jews, Gypsies, Communists,
Catholics, homosexuals, and political opponents of the government. During the 1930s and 1940s, a large portion of the state's resources funded a system of death camps which, by the end of World War II, exterminated nearly twelve million people.
Behavioral Genetics Today
Galton's association with the eugenics movement often overshadows the fact that he made major contributions to the study of human genetics and statistical analysis of traits. It was Galton who first suggested some of the major methods of human behavioral genetics used today. He conducted the first systematic family, twin, and adoption studies to show how behavioral traits can run in families. In addition, he devised the concept of correlation, an important tool used to describe resemblances among family members and a fundamental concept of statistical analysis today.
Largely tainted by the ideas of the eugenics movement, twin and adoption studies conducted during the 1920s and 1930s played only a minor role in the advancement of behavioral genetics. During the same period, studies demonstrating the importance of genetics on the behavior of animals actually had greater impact on the development of today's behavioral genetics. Scientists carried out selection experiments to create breeds of rats, mice, and dogs characterized by particular behavioral traits such as curiosity and emotionality.
In the 1960s, studies on the genetic influence of traits such as IQ once again gained notoriety, but their results quite often remained ignored because of their associations with eugenics and racism. It was not until the 1980s that the scientific community began to pay serious attention to behavioral genetics as a science. Using gene manipulation technologies which had been in the works since the late 1960s, geneticists could finally map the human genome itself.
Since the early ’80s, geneticists have identified genes for cystic fibrosis, muscular dystrophy, and Huntington's disease. The successes in identifying and mapping human disease genes have revitalized interest in identifying genetic factors underlying behavioral traits. Twin and family studies, genetic studies of animal behavior, biochemical investigations of mutations, and the development of new molecular approaches, have all added to the momentum of the modern search for genes influencing behavior.
Genes and Aggression
Is the propensity for violence inherited?
During the Bush administration, the U.S. government announced a plan to begin a
"violence initiative" to find the causes-genetic and otherwise-of violence. While some

studies did receive funding, the initiative as a federal program was scuttled at the last minute largely due to protests that it had a racist intent.
Some studies have, nonetheless, pointed to a genetic basis for violence. In 1993, investigators in the Netherlands reported the discovery of what seemed to be an aggression gene, perhaps akin to that which Stephen Mobley claims to run in his family.
Biochemical evidence supported the link between the particular mutant, or altered, gene and aggressive behavior.
The gene coded for an enzyme involved in the metabolism of certain chemicals that transmit signals within the brain and nervous system. Urinalysis of subjects in the Dutch study, all of whom were related and demonstrated aggressive and antisocial behavior, showed abnormal levels of metabolic products associated with the enzyme. The affected individuals lacked the ability to produce the enzyme, and this deficiency may have led to their antisocial behavior.
Gay Genes?
Well-controlled investigations with Drosophila melanogaster, a common fruit fly species, have suggested that a multitude of genes contribute to most behaviors. In a
1992 study, researchers identified Drosophila genes that affect learning, motor behavior, and courtship and mating. Abnormal expression of one gene causes male flies to court other male flies instead of females. Could this finding suggest that genetic factors might determine homosexuality in humans?
Twin studies of homosexual men and women suggest a strong genetic component in homosexual behavior. The experimental design of twin studies is based on the fact that identical twins are genetically matching while fraternal twins are only 50 percent identical. Therefore, identical twins should share inherited traits more often than do fraternal twins. For example, height yields an identical twin correlation of about 0.90 and a fraternal twin correlation of about 0.45, indicating that genetic factors must indeed influence height. Similar experiment designs have yielded significant heritability values for homosexuality.
Work using Restriction Fragment Length Polymorphism (RFLP) markers to correlate the presence of particular gene forms with homosexuality has indicated links between male homosexuality and genetic markers on the X chromosome. Researchers, however, have yet to identify a specific gene for homosexuality.
Other scientists have found evidence that responses to alcohol are heritable in both humans and animals and that multiple genes determine this sensitivity. Some researchers believe a gene that encodes a neurotransmitter receptor protein might contribute to alcoholism. Some studies have found a high frequency of one allele, or variation, of the gene in alcoholic men. It has, however, proved difficult to prove a link to alcoholism.
In addition, scientists have found a gene thought to confer susceptibility to schizophrenia in the genome of a substantial number of those who develop this disorder. Making correlations between the disease and gene frequencies in affected individuals and pinpointing the precise location of the problem genes in the genome still leaves researchers confused.

Scientists are even studying the roles of genes in human tendencies and traits such as depression and intelligence. Because such studies are quite controversial, many people have chosen to dismiss the findings. Almost all the studies of these complex human conditions and qualities have provided only indirect and correlative-not causativeevidence for the roles of specific genes. The evidence suggests that many genes influence such conditions but that linking them to specific genes will prove difficult.
Environment and genes
Though we have traditionally phrased the debate about human behavior as "nature versus nurture," it is perhaps more appropriate to call it "nurture given nature" to determine the importance of the environment in the expression of genes without diminishing the important role of genes themselves. Such an encompassing approach has led to novel ways of thinking about the biology of human behavior. Cornell psychology professor Daryl Bem's "exotic becomes erotic," or EBE, theory of the development of human sexual orientation is a good example of consideration of environmental factors in explanations of human behavior.
Bem maintains that adult sexual orientation is actually rooted in childhood preferences for gender-typical or gender-atypical activities and friends. Children who prefer gender-typical activities and friends eventually come to perceive members of the opposite sex as different; this sense of unfamiliarity translates into an exotic view of the opposite sex. Children preferring gender-atypical activities and friends ultimately view members of their own sex as different or exotic. During puberty, feelings of being different evolve into sexual attraction. Thus, what is exotic becomes erotic. Although
Bem does not deny the existence of genetic influences on sexual preferences, his theory clearly suggests that heterosexuality and homosexuality are highly determined by an individual's own environment.
Of course, one may create a perpetual argument that it is ultimately an individual's own choice-whether conscious or not-to incorporate the effects of environmental factors into his or her personality or behavior patterns, and that one's propensity to make any such choice is largely founded in genetic makeup. Indeed, the debate about what factors truly contribute to one's behavior will likely continue for a long time to come.
As more research attempts to reveal the precise relationship between genes and behavior, scientists will be presented with a new challenge: to present and discuss their results along with the meanings and implications of such results. The study of behavioral genetics treads on delicate ethical grounds. As has happened in the past, behavioral genetics research data interpreted and usurped by bigoted people easily might be used in the future for a new wave of eugenic thought, leading to extensive discrimination and injustice. To some, reducing behavior to the molecular level may be an offensive oversimplification of human nature-a true blow to personal, religious, and societal concepts of free will and fate. In the end, those who choose to take up the search for intrinsic causes of our behavior will certainly need to evaluate the effects their work will have on society.
Marcelo Vinces graduated from Cornell