Genetics of aggression

The field of psychology has been greatly influenced by the study of genetics. Decades of research has demonstrated that both genetic and environmental factors play a role in a variety of behaviors in humans and animals (e.g. Grigorenko & Sternberg, 2003). The genetic basis of aggression, however, remains poorly understood. Aggression is a multi-dimensional concept, but it can be generally defined as behavior that inflicts pain or harm on another.

Genetic-developmental theory states that individual differences in a continuous phenotype result from the action of a large number of genes, each exerting an effect that works with environmental factors to produce the trait. Because this type of trait is influenced by multiple factors, it is more complex and difficult to study than a simple Mendelian trait (one gene for one phenotype).

History
Past thought on genetic factors influencing aggression tended to seek answers from chromosomal abnormalities. Specifically, four decades ago, the XYY genotype was (erroneously) believed by many to be correlated with aggression. In 1965 and 1966, researchers at the MRC Clinical & Population Cytogenetics Research Unit led by Dr. Court Brown at Western General Hospital in Edinburgh reported finding a much higher than expected nine XYY men (2.9%) averaging almost 6 ft. tall in a survey of 314 patients at the State Hospital for Scotland; seven of the nine XYY patients were mentally retarded. In their initial reports published before examining the XYY patients, the researchers suggested they might have been hospitalized because of aggressive behavior. When the XYY patients were examined, the researchers found their assumptions of aggressive behavior were incorrect. Unfortunately, many science and medicine textbooks quickly and uncritically incorporated the initial, incorrect assumptions about XYY and aggression—including psychology textbooks on aggression.

The XYY genotype first gained wide notoriety in 1968 when it was raised as a part of a defense in two murder trials in Australia and France. In the United States, five attempts to use the XYY genotype as a defense were unsuccessful—in only one case in 1969 was it allowed to go to a jury—which rejected it.

Results from several decades of long-term follow-up of scores of unselected XYY males identified in 8 international newborn chromosome screening studies in the 1960s and 1970s have replaced pioneering but biased studies from the 1960s (that used only institutionalized XYY men), as the basis for current understanding of the XYY genotype and established that XYY males are characterized by increased height but are not characterized by aggressive behavior. Today the link between genetics and aggression has turned to a different aspect of genetics than chromosomal abnormalities but it is important to understand where the research started to understand the direction it is moving towards today.

Heritability
Aggression, as well as other behavioral traits, is studied genetically based on its heritability through generations. Heritability models of aggression are mainly based on animals due to the ethical concern in using humans for genetic study. Animals are first selectively bred and then placed in a variety of environmental conditions, allowing researchers to examine the differences of selection in the aggression of animals.

Research methods
As with other topics in behavioral genetics, aggression is studied in three main experimental ways to help identify what role genetics plays in the behavior:
 * Heritability studies – studies focused to determine whether a trait, such as aggression, is heritable and how it is inherited from parent to offspring. These studies make use of genetic linkage maps to identify genes associated with certain behaviors such as aggression.
 * Mechanism experiments – studies to determine the biological mechanisms that lead certain genes to influence types of behavior like aggression.
 * Genetic behavior correlation studies – studies that use scientific data and attempt to correlate it with actual human behavior. Examples include twin studies and adoption studies.

These three main experimental types are used in animal studies, studies testing heritability and molecular genetics, and gene interaction/environment studies. Recently, important links between aggression and genetics have been studied and the results are allowing scientists to better understand the connections.

Selective breeding
The heritability of aggression has been observed in many animal strains after noting that some strains of birds, dogs, fish, and mice seem to be more aggressive than other strains. Selective breeding has demonstrated that it is possible to select for genes that lead to more aggressive behavior in animals. Selective breeding examples also allow researchers to understand the importance of developmental timing for genetic influences on aggressive behavior. A study done in 1983 (Cairns) produced both highly aggressive male and female strains of mice dependent on certain developmental periods to have this more aggressive behavior expressed. These mice were not observed to be more aggressive during the early and later stages of their lives but during certain periods of time (in their middle-age period) were more violent and aggressive in their attacks on other mice. Selective breeding is a quick way to select for specific traits with the effects of selection being seen within a few generations of breeding. These characteristics make selective breeding an important tool in the study of genetics and aggressive behavior.

Mouse studies
Mice are often used as a model for human genetic behavior since mice and humans have homologous genes coding for homologous proteins that are used for similar functions at some biological levels. Mice aggression studies have led to some interesting insight in human aggression. Using reverse genetics, the DNA of genes for the receptors of many neurotransmitters have been cloned and sequenced, and the role of neurotransmitters in rodent aggression has been investigated using pharmacological manipulations. Serotonin has been identified in the offensive attack by male mice against intruder male mice. Mutants were made by manipulating a receptor for serotonin by deleting a gene for the serotonin receptor. While exhibiting normal behavior in everyday activities such as eating and exploration, these mutant male mice with the knockout alleles attacked intruders with twice the intensity of normal male mice. In offense aggression in mice, males with the same or similar genotypes were more likely to fight than males that encountered males of other genotypes. Another interesting finding in mice dealt with mice reared alone. These mice showed a strong tendency to attack other male mice upon their first exposure to the other animals. The mice reared alone were not taught to be more aggressive; they simply exhibited the behavior. This implicates the natural tendency related to biological aggression in mice because the mice reared alone lacked a parent to show them when to be aggressive.

Biological mechanisms
In exploring how aggression is influenced by genetics, experiments designed to study biological mechanisms are utilized. Molecular genetics studies allow many different types of behavioral traits to be examined by manipulating genes and studying the effect of the manipulation.

Molecular genetics
A number of molecular genetics studies have focused on manipulating candidate aggression genes in mice and other animals to induce effects that can be possibly applied to humans. Most studies have focused on polymorphisms of serotonin receptors, dopamine receptors, and neurotransmitter metabolizing enzymes. Results of these studies have led to linkage analysis to map the serotonin-related genes and impulsive aggression. In particular, the serotonin 5-HT seems to be an influence in inter-male aggression either directly or through other molecules that use the 5-HT pathway. 5-HT normally dampens aggression in animals and humans. Mice missing specific genes for 5-HT were observed to be more aggressive than normal mice and were more rapid and violent in their attacks. Other studies have been focused on neurotransmitters. Studies of a mutation in the neurotransmitter metabolizing enzyme monoamine oxidase A (MAO-A) have been shown to cause a syndrome that includes violence and impulsivity in humans. Studies of the molecular genetics pathways are leading to the production of pharmaceuticals to fix the pathway problems and hopefully show an observed change in aggressive behavior.

A rare genetic variant causing MAO-A deficiency has been associated with violent behavior in males. In 2002 a study published by researchers at King's College London found a link between a genetic variant causing low levels of MAO-A and increased levels of antisocial behavior in people who had been mistreated as children. An American group studying monkeys called MAO-A a "warrior gene" in 2004. A 2008 study found a similar result involving the variant of the MAO-A gene, as well as the genes DAT1 and DRD2. In all three cases, the variants of these genes were associated with an elevated risk of violent and delinquent behavior, but only in people who experienced certain stresses during childhood.

Human behavior genetics
In determining if a trait is related to genetic factors or environmental factors, twin studies and adoption studies are used. These studies examine correlations based on similarity of a trait and a person's genetic or environmental factors that could influece the trait. Aggression has been examined via both twin studies and adoption studies.

Twin studies
Twin studies manipulate the environmental factors of behavior by examining if identical twins raised apart are different from twins raised together. Before the advancement of molecular genetics, twin studies were almost the only mode of investigation of genetic influences on personality. Heritability was estimated as twice the difference between the correlation for identical, or monozygotic, twins and that for fraternal, or dizygotic, twins. Early studies indicated that personality was fifty percent genetic. Current thinking holds that each individual picks and chooses from a range of stimuli and events largely on the basis of his genotype and creates a unique set of experiences, basically meaning that people create their own environments.

Adoption studies
Adoption studies allow genetic factors of behavior to be tested by taking advantage of the fact that an adopted person shares their genetic makeup with their biological parents but was actually raised by people other than the biological parents. Therefore, if a behavior is genetic the person should resemble their biological parent but if a behavior is more environmentally influenced, the person will resemble the parent that raised the person. One adoption study in Denmark found that adoptive siblings split into separate foster homes had a 12.9% concordance rate of crime and aggressive behavior. This study also showed correlations between biological parents who committed criminal activity and their children they gave up for adoption committing criminal activity. This seems to suggest that biological characteristics which increase the risk for criminal convictions and aggression are transmitted from biological parents to their offspring and increase risk for criminal convection. Though many believe that new technology is making twin and adoption studies seem less relevant they are still important in assessing the role of environmental factors versus genetic factors in such characteristics as aggression.

Future research
The explosion of genetic discoveries and the thinking of a new generation of scientists and social scientists have allowed genetics and aggression to be linked together without as much controversy as was characteristic in the past. Much of the current research involving genetics and aggression is aimed at understanding the link between genes and the environment that leads to aggression. Psychosocial researchers are now using genetic information as a control variable in experiments allowing them to be better able to study the environmental effects on aggression. Current thinking is that eventually studies investigating the effectiveness of aggression prevention programs will test genotypes of participants and identify those who are at "more risk" towards aggression. These more at risk people will be compared to less at risk individuals to examine how well prevention programs can combat genetic aggression. For now it is safe to say that studies exploring the genetic factors of aggression will still be mostly confined to animal studies as the ethics of human studies is still a debate.

Many species of animals still need to be studied to better understand genetics of aggression. At the moment, studies on monkeys, fish, and small mammals are being conducted and will utilize genetic information and understanding to better understand the role the environment plays alongside genetics in the development of aggression. These comparative genetics studies will allow researchers to compare similar traits of aggression in many species and also identify special aggressive traits specific to certain animals all allowing for a better model of human aggression.