There is an old cliché that goes “the apple doesn’t fall far from the tree.” While this is usually used to describe the relationship between a parent and a child, it can also be used to describe the behavior that child exhibits. With the constant amount of research that revolves around criminalistics behaviors, the nature versus nurture debate has gained much steam. While some say criminal behaviors are acquired through one’s surroundings, other say that they are acquired genetically. In a genetic sense, the apple really does not fall far from the tree. The genes an individual is born with are the foundation of their actions, emotions, and thought process. There are a multitude of studies that prove that criminal behavior is predominantly acquired through one’s genetic makeup, which can be characterized through genetic markers on the MAOA and CDH13 genes, reductions in the development of the prefrontal cortex, low blood pressure and obstetric factors.

There are numerous case studies revolving around twins in regards to criminal behavior. The reason for that is twins have almost identical genetic makeups. Identical, or monozygotic, twins have nearly indistinguishable genetic DNA structure, whereas fraternal, or dizygotic, twins share around half of their genetic makeup. *The nearly identical surroundings and genetic makeup of monozygotic twins make them perfect candidates for displaying the relationship that genetics has on criminal behavior. Any differences in behavior are assumed to be because of non-shared environmental factors, such as different friend groups or  uneven parental treatment. (Rutter)

The first twin study is from the Vietnam Era Veteran Twin Registry. The study interviewed 3,226 pairs of male twins from ages 36 to 55 years old, and all ethnic backgrounds,* and who served in the military during the Vietnam War. Of the twins interviewed, over half were monozygotic. The questions asked revolved around arrest records, felony conviction, and early-late criminal behavior(Rutter). The study found that “neither later arrests nor later criminal behavior are significantly influenced by the common environment, but both are significantly heritable” (Rutter). They came to this conclusion by comparing the twins answers to various questions and seeing if they had similar criminal background in order to help prove a genetic relationship with criminal behavior (Rutter). This study proves that at least amongst males, there is a stronger correlation between criminal behavior after the age of 15 and genetics than criminal behavior and environmental factors. This can be seen through the lack of effect that shared environment was found to have on criminal behavior and the large effect heredity was found to have.

In a YouTube interview of Adrian Raine, a well-known psychologist, Raine states that “twin studies on their own show that at least 50 percent of the cause of crime and antisocial behavior can be chalked up to genes” (Raine). The other 50 percent is split between shared environmental factors, which account for little cause of criminal behavior, and non-shared environmental factors, which make up the rest. Non-shared environmental factors amongst twins would be considered different friend groups or uneven parental treatment. This results in the biological or genetic component making up the largest portion especially when you do not combine the two very different types of environments. One of the main issues with the twin studies is that some of the assumptions made on environmental factors are incorrect, specifically * that monozygotic and dizygotic have the same amount of similarities in environmental factors. Identical twins, however, are more similar on all levels, including an environmental level. Because of this, some believe twin studies might overestimate the genetic impact on criminal behavior. Twin studies also only represent a small portion of the population. That is why a study on siblings was used to correct some of the possible underestimating of non-shared environmental factors from the twin studies, as well as give a better representation of the whole population.

In order to further support and prove the findings in twin studies, a study involving full and half siblings was conducted. This specific study tested both female and male sibling pairs of either full siblings living together, half siblings living together or half siblings living apart. All of the subjects were born between 1958 and 1991, with no more than a 10-year age gap between the siblings. All the pairs were compared to the Swedish crime registry. The study made sure to account for shared environmental factors, such as family and shared friends. It also included non-shared environmental factors, such as one sibling having developmental problems and non-shared acquaintances. Any error was also included in the non-shared environmental factors. The results are as followed: “Using a range of specific models applied to full- and half-sibling pairs, we found that genetic risk factors contributed substantially to the etiology of CB, with heritability estimates ranging from 33% to 55% in females and 39% to 56% in males. Across all models, shared environmental factors were considerably less important than genetic influences” (Kendler). This sibling study also found that “estimates for the shared environment varied from 1% to 14% in females and 10% to 23% in males, which is increasing lower than those estimated in the twin study” (Kendler). This shows that even on the lower end of the percent range, genetics are still considered to be sizably more responsible for criminal behavior then even the highest end of the shared environmental factors percent range. It is also uncertain, and not mentioned in the study, what percent non-shared environmental factors contributed to criminal behavior. This may be because non-shared environmental factors were lumped in with experimental error, as well as there is no way of knowing all the non-shared environmental factors involved. It also showed that genetic factors have the largest influence on criminal behavior, with a potential influence of more than 50 percent. * 

A lot of people will say that there is not a “crime” gene, which is correct. Not any one gene can be entirely to blame for someone’s behavior, but that does not mean that there are not any genes linked to criminal behavior. A study of about 900 criminal offenders in Finland has recently found a trend in their genetic makeup. A large number of these criminals seem to have two common genes, one of the genes, the MAOA gene, commonly known as the “warrior gene,” is linked to aggressive and antisocial behavior, both of which lead to criminal behavior as seen amongst the most violent of the 900 criminal offenders. The other gene, the CDH13 gene, which is known for its association with substance abuse, is also known to lead to criminal behavior. Studies show that “those with the genes were 13 times more likely to have a history of repeated violent behavior” (Hogenboom). This statistic shows alarming evidence of genetic factors that are associated with criminal behavior that can not be influenced or affected by environment in any form.

Adrian Raine *has conducted extensive research in this field and has been studying criminals for years. One of his largest discoveries came from a study conducted on murders in  American prisons. The study compared the brain scans of 41 murderers to brain scans from 41 non convicted adults within the same age range and profile. What was observed was that “the murderers' brains showed what appeared to be a significant reduction in the development of the prefrontal cortex, ‘the executive function’ of the brain, compared with the control group” (Adams). An under-developed prefrontal cortex can result in risk taking and poor problem-solving skills, which are both commonly seen in criminal offenders. This provided a specific genetic component common amongst criminals that could be in part to blame for their behavior.

In addition to Raine’s study of criminals’ brain scans, he has also found other factors that seem to have a correlation with criminal behavior: obstetric factors*. For example, some minor physical abnormalities caused by pregnancy complications that Raine talks about in Biosocial Studies of Antisocial and Violent Behavior in Children and Adults: A Review are “low-seated ears, adherent ear lobes, and furrowed tongue” (Raine 1). All of the physical abnormalities Raine mentions have been linked to antisocial and violent behavior.

There have also been a lot of studies that can link birth complications to delinquency and violence. Specifically, the birth complication of an underweight infant could directly cause damage to the fetus brain similar to damage observed during his study of 41 murderers’ brain scans. Malnutrition as an infant can also be considered an obstetric factor that has been previously linked to antisocial and violent behavior. Visual signs of malnutrition can be a child's cracked lips and thin, or discolored, hair (Liu). *Another large obstetric factor is whether or not the mother drank alcohol or smoked during her pregnancy. Both of these habits have been directly linked to conduct disorders and violent behavior in adolescence. A study of adult males showed there to be a twice as large increase in whether someone engaged in violent criminal behavior if their mother was a routine smoker. It also found a direct link between an increased number of cigarettes smoked by the mother and an increased level of violence displayed by the son (Raine, Biosocial Studies of Antisocial and Violent Behavior). The most shocking of these statistics being a “fivefold increase in adult violence was found when nicotine exposure was combined with exposure to the delivery complications” (Raine, Biosocial Studies of Antisocial and Violent). What this study proves is that a child’s behavior is directly linked to how their mother behaved while pregnant, which can be seen as a biological factor completely out of the child’s control that ultimately can lead to violent and criminal behavior. This study also shows a large increase in the likelihood of engaging in criminal behavior the more obstetric factors the child has.

Another factor that has shown up in Raine’s work and various other studies is a correlation between criminal behavior and low resting heart rate as an adolescent. A study was conducted on 5,000 children in Europe born in March of 1946, who had their heart rate measured at age 11. The study showed that “81 percent of violent offenders and 67 percent of sexual offenders had below-average heart rates” (Raine, Biosocial Bases of violence ). The Cambridge Study in Delinquent Development produced similar results as it measured 411 London boys resting heart rates at age 18. The results of which showed that “the boys who were convicted of violence before age 25, and those who were chronic offenders (with six or more convictions), had significantly lower heart rates” (Raine, Biosocial Bases of violence). Resting heart rate should be seen as another genetic factor directly linked to criminal and violent behavior. * This does not, however, suggest that all people with low resting heart rates are criminal offenders, rather it strictly means that a lot of criminal offenders have the genetic mutation of a low resting heart rate.

Raine’s research would prove that for some of the more violent criminal offenders, there is a prominent genetic component. Despite this being strong evidence to support biological factors playing the largest role on criminal offenses, there is one flaw. Raine chose to study his own brain in addition to others and realized he had more similarities with the murderer’s brain scans than the control group. In addition to that, he was “born at home; he was a blue baby, always had a markedly low heart rate, and as plagued by cracked lips as a child” (Adams), all of which are markers and red flags for criminal behavior. However, although Adrian Raine is not currently a criminal, he did engage in a variety of criminal behavior when he was an adolescent, ranging from setting fires to drinking so much alcohol that he had to have his stomach pumped (Adams).

Adrian Raine does not share a notable distinguishing factor amongst criminals: low IQ. There has been a series of studies conducted on the matter that came to the conclusion that criminal brains have lower IQ’s. One study in particular found that “the IQ’s of juvenile delinquents were significantly lower than the IQ’s of comparison groups. Prospective longitudinal studies have also reported that the childhood IQ’s of juvenile delinquents were lower than the childhood IQ’s of non-delinquents” (Kandel). One unpopular theory supporting the association of low IQ’s and criminal behavior is that criminal offenders with higher IQ’s are less likely to get caught, and therefore are not included in these studies. However, this theory is usually refuted by the large amounts of statistical evidence to support that low IQ is in fact associated with criminal behavior. 

Despite all the information pointing to Raine being a criminal, he is not. He is actually *a successful psychologist and professor. However, this does not disprove the idea that biological factors prominently impact one’s likelihood to commit criminal behavior. It merely states the environment or additional factors that contribute to this idea. Raine was also missing other key factors that have been found in criminals, such as low IQ. He should be thought of as the exception to the rule. Raine himself even states that he engaged in a lot of delinquent behavior when he was younger. He was “antisocial, in a gang, smoking, letting car tires down, setting fire to mailboxes, and fighting a lot” (Adams). All of which make sense based on his genetic makeup, but how he was able to beat the odds and pull himself out of the rebellious behavior is probably due to environmental circumstances. Professor Raine’s genetics might also explain his career choice, and why he wants to study the brains of criminals. His own genetics most likely lead him to his career, and he quite possibly could have wanted to study likeminded people. (Adams).

One of the strongest pieces of evidence that supports a large genetic component to criminal behavior can be found through adoption studies. The reason for using adoption studies is because they are able to prove hereditary relationships that are unaffected by parental relationships with that person, since there most likely not in one. Parent-child relationships are considered environmental factors and the topic of adoption eliminates that environmental factor from the equation.  A Swedish Adoption Study took place in the mid nineteen hundreds which consisted of 862 men and 913 women. All participants were born in the early nineteen hundreds between the ages of 20 to 40 years old during the time of the study in order to try to minimize the effect that environmental factors may have on the results. * The candidates selected for this study were placed into equally economically stable adoption homes at young ages, and in addition, the adoptive parents had zero criminal or alcohol dependency after running background checks. As it turns out, a large percentage of children whose biological parents were alcohol abusers also ended up abusing alcohol in their later adult years (Rutter). Although this is not directly dealing with criminal behavior or acts of violence, it does prove a strong correlation between genetics and alcohol abuse similar to antisocial behavior. Alcohol and other drug abuse as well as antisocial behavior have been strongly affiliated with criminal behavior.

A *similar adoption study was conducted in Northern Europe which combined the conviction records of over 14,400 teenagers adopted out of family ages 15 years and older. This study then compared those records to the conviction records of their adoptive and biological parents in order to find a correlation in behavioral patterns that would further prove a comparable genetic factor. Information concluded on the relationships found goes as followed: “if neither the biological nor the adoptive parents are convicted, 13.5 percent of sons are convicted. If the adoptive parents are convicted and the biological parents are not the percent only rises to 14.7 percent. However, if the adoptive parents are not convicted and the biological parents are, 20 percent of sons are convicted” (Sarnoff). This information proves that in general genetics play a larger role on whether or not criminal offenders offspring will follow in their delinquent footsteps. The biological parents and the adoptees do not share any common environmental factors. What this means all of the correlation between adoptive children’s criminal activities and the biological parents criminal activities can only be categorized as genetic. The information collected also shows that the behavior of the adoptive parents, which would be considered an environmental factor, has a minimal effect on their adopted child’s behavioral patterns. According to this study, the adoptive child’s chances of committing a crime are nearly the same, regardless of their adoptive parent’s criminal behavior (Sarnoff ).

The study also found that in the specific cases referring to property crimes, it was found that there is a strong correlation between adoptees and their biological parents committing similar crimes. However, the same could not be said for violent crimes where there was no correlation. Along with that information, there was also little to no correlation between adoptees conviction records and their adoptive parent’s conviction records (Sarnoff). This shows that for specifically smaller, less severe crime cases, genetics do play a substantial role with little to none of this correlation relating directly to environmental factors. Once again this statistic is only true for biological parents and there is found to be no correlation between smaller crimes such as property theft and adoptive parent’s behavior(Sarnoff).

Genetics is as one may say “the be all and all” of criminal behavior. What this means is that it is the most important factor relating to the cause of criminal behavior. This can be seen through the twin studies that found that genetics, on the high end, can account for 55 percent of criminal behavior. The other half, by default, has to be divided by shared and non-shared environmental factors. It can also be seen through the strong correlation in criminal behavior between adoptive kids and their biological parents, as well as the non-existent correlation of criminal behavior between adoptive parents and their adoptive children. However, that is not to say that environmental factors do not play a role in criminal behavior, rather that they should just be considered the “straw that breaks the camel’s back”. Someone will behave criminally if all of their genetics point to criminal behavior, with the help of a little push from environmental factors. *

The odds that one will demonstrate criminal behaviors dramatically increases depending on their genetics. Studies involving twins, adopted children and siblings have not only been able to prove a strong correlation between criminal behavior and genetics, but also show that genetics makes up majority of influencing factors. While there is not a specific genetic makeup that can guarantee a person will grow up and turn to criminal activities, there are certain genetics that can be a red flag and makes an individual more susceptible to committing crimes. Certain genetic markers that correspond with criminal behaviors are the MAOA and CDH13 gene, reductions in the development of the prefrontal cortex, low blood pressure and obstetric factors, all of which can act as red flags to those studying human behavior, genetics and criminal activity. 
