When we are confronted with the acts of excessive and unprovoked violence, we can’t help but wonder what is wrong with individuals committing such crimes. Think of serial killers: what motivates them? Both researchers and society, in general, have wanted to know how to explain the extreme brutality observed in some people. In most cases, they have no mental disorders that could explain their behavior. Even without going to the extremes, most of us did at some point in life come across people whose level of aggressiveness seemed beyond any reasonable explanation. Think of a hooligan looking for any excuse to pick a fight and beat someone up. Or a young boy torturing a defenseless animal with a smile on his face. Multiple theories were invented to this end, ranging from religious explanations (satanic possession) to scientific and psychological theories that involve a variety of mental disorders or problems with brain development.
The idea that excessive aggressiveness and criminal tendency might be heritable traits gained popularity with the publication of Dr. Cesare Lombroso’s book “Criminal Man” in 1878. In the book, Lombroso introduced the concept of the “born criminal”. He also developed the field of criminal anthropology that studied specific anatomical differences between normal and criminal individuals. Lombroso’s theory contributed to the science of eugenics that played a crucial role in the Nazi ideology of selective breeding of a superior race and the policy of exterminating the Untermenschen.
Although Lombroso was eventually proven wrong, the concept that criminal behavior might be linked with genes survived. Evidence that criminal and violent behaviors run in some families was a particularly strong argument to investigate the issue further. These investigations produced rather interesting discoveries.
The question to what degree the predisposition for crime might be genetically determined was first answered by a twin study performed in Denmark. Twins are ideal subjects for genetic research: identical twins have exactly the same sets of genes, while non-identical twins are as similar to each other as usual brothers and sisters. However, both identical and non-identical twins, if brought up together, can be considered as having the same upbringing. The study compared the rate of crime offenses among the identical twins with this rate in non-identical twins. It turned out that a Danish man with an identical twin who has a criminal record is 50% more likely to be an offender himself, as compared with the average Danish man. In non-identical twins, the chances of both of them having the criminal records are 15-30% higher than the average for the population. The findings definitely point to a degree of genetic predisposition. In addition, another study performed in Sweden has shown that when the identical twins were brought up separately, the chances of developing a criminal career were higher among children from parents with criminal records, even when the children were brought up in law-abiding adopted families.
Twin studies can detect correlations but certainly can’t help in finding out which genes are behind these correlations. The study performed in the Netherlands provided important information on the possible identity of such genes. Researchers have studied genetic defects in one particular family with 14 males spanning 4 generations that displayed an unusually high level of aggression and criminal offenses. The subjects in question had very low IQ (around 85) and were prone to impulsive behavior and physical and sexual violence. The researchers found a specific hereditary defect in the family: the gene for monoamine oxidase A (MAOA) was mutated. Mutation prevented the enzyme from working properly. This is important as this enzyme is responsible for breaking down neurotransmitters, including serotonin, dopamine, and noradrenaline. A lack of MAOA activity leads to the rising of neurotransmitter levels in the brain and they, in turn, cause the over-excitation of neurons. The gene for MAOA is located on the X chromosome, and this explains why high levels of aggression were observed only in males. Meanwhile, females have a second X chromosome with the non-mutated functional version of the gene.
An important question, which sparked fierce ethical debate, is to what extent criminal behavior might indeed be genetically programmed. This is a classic discussion of nature vs nurture. To what extent do our genes make us who we are? We easily accept the fact that some people are born smarter or physically stronger than the rest of us. We know that genes are involved in making these individuals who they are. Genes responsible for stronger muscles or better brain connections allow these people to excel where others may struggle. Nonetheless, the idea that some of us are born with a predisposition for a higher level of aggression or reduced empathy appears very unpalatable to many people. However, this idea makes perfect biological sense. We evolved as hunter-gatherers, and at this stage of our evolutionary history, aggressiveness was crucially important for survival. Genetically, we didn’t change since the Stone Age. And this aggressiveness still plays an important role in our society, from competition in the workplace to multiple armed conflicts around the world. Aggression levels, like many other human behavioral traits, can be genetically determined to a degree. This means that there is variability: in some people, the level of aggressiveness is very low, while in others it can be quite high.
Aggressiveness still doesn’t equal crime: although violent crime requires a perpetrator to be aggressive, the two things are not the same. Social factors still play a key role when it comes to the expression of aggressive behavior. It works the same way with other genetic attributes. A born athlete will never reach his Olympic dream and could turn into a couch potato if they don’t train. Most scientists, even the very successful ones, are not born geniuses: they simply worked and studied hard. Similarly, people with a predisposition for higher levels of aggression are at higher risk of becoming criminals when they are exposed to the social factors that lead them in that direction.
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