Nature vs. nurture, the question that has vexed psychology since the discovery of inherited traits. We know that nature is to be blamed for physical traits such as hair color, eye color, even some diseases, but we would not say that these things are what make people who they are. All people with blond hair are not the same, nor are all of the people in the same family always blond-haired or blue-eyed. Siblings may very well have different interests, different habits, and even different appearances. So how much of whom we are is determined by our genes and how much is dependent on our environment?
There is significant controversy surrounding the debate of Nature vs. Nurture. The bulk of this controversy lies in that, in order to perform a flawless experiment regarding this, would inherently be viewed as unethical.
Even siblings, with the exception of identical siblings, inherit different genes from their parents. Therefore, what one child may inherit, another may not. Even if two children do inherit the same gene for a specific characteristic, other factors such as birth order, individual experiences, and different treatment by parents or teachers may skew any scientific results. For example, only a first or only child can ever experience infancy without siblings, a child must be an only child or the last child to never experience a younger sibling, and children of different ages will share experiences with different friends, different teachers, and different places. Therefore, it is only natural that all people are different, and it is very difficult to determine whether it is genetics or environmental differences that cause these discrepancies.
Now, with a newer concept that who a person becomes is a result of environmental influences on genetics, and is neither a result of genetics nor environment alone, the controversy is compounded. It is no longer simply a question of nature or nurture. We now have a third option, nature AND nurture.
In order to perform a definitive study on the subject of nature and nurture, you would need several large groups of subjects that were genetically identical, and several identical environments in which each subject experienced the exact same events and treatment without outside influences. Then, in order to ensure that the subjects were not influenced by the study itself or other test subjects, the subjects would need to be uninformed that the study was occurring and uninformed of the existence of their identical siblings or clones. These conditions can never be met ethically.
Peter Neubauer, a child psychiatrist, and Viola Bernard, a child psychologist performed an almost perfect study on this subject from 1960-1980. At the time, according to Neubauer, it was believed that identical twins should be separated at birth to allow them to develop individual personalities (no evidence of this concept was ever found). Their study took 13 infants that were identical twins and triplets that were due to be put up for adoption and enrolled them in study that would follow them as they grew up with different adoptive parents. Since identical siblings have identical DNA, this would appear to be the perfect way to officially determine how much of who we are is biological and how much is environmental. However, due to concerns over public opinion, the records of this study will remain sealed in a vault at Yale University until 2066.
Until the records from Neubauer’s study are released, we will have to rely on other studies, performed more recently. Most of these newer studies focus on more specific traits such as specific diseases, intelligence and criminality.
“The Swedish Twin study of CHild and Adolescent Development (TCHAD) is a longitudinal study of how genes and environments contribute to development of health and behavioral problems from childhood to adulthood.”
The Swedish Twin study of CHild and Adolescent Development, or TCHAD-Study is one of the largest twin studies regarding the Nature vs. Nurture debate thus far. It gathered data from 2960 subjects, 1480 pairs of twins and their parents, over the past 16 years, with the last data recorded in 2005, when the twins were all between the ages of 19 and 20. While this study is longitudinal, and on-going, much data has already been recorded, and much has been learned.
The focus of this study has been to look at externalizing symptoms, such as ADHD, criminal behavior and psychopathic personalities. However, many internalizing relationships have also been tracked, such as life events, relationships, and different clinical assessments that could impact the externalizing symptoms.
The data for this study was collected at four separate times between 1994 and 2005, when the children were between the ages of 8-9, 13-14, 16-17, and 19-20. Demographic information for the children was researched through their mailing zip codes and birth weight was tracked through the National Birth Register. The Achenbach System of Empirically Based Assessment (ASEBA) measures were used to collect progressive information, such as competence, behavior, and personality about each of the children and their parents through questionnaires and surveys sent through the mail and phone interviews.
While the study is on-going, the conclusions that have already been reached are significant. Among these findings are:
Aggressive behavior is highly heritable in childhood, but becomes significantly affected by environment and genetics with age.
Non-Aggressive behavior is affected more by environment, especially as a person ages.
Anti-social behavior’s heritability increases with age for girls and decreases with age for boys.
ADHD is highly heritable, but onset may be impacted by environment
Psychopathic behavior is one of the most heritable traits found in this study
None of these traits are 100% heritable or environmental. Therefore, it is safe to conclude that, while genetics play a significant role in personality and behavior disorders, environment also plays a significant role. For example, while aggressive behavior is highly heritable and highly stable over time, not all children of aggressive parents will be aggressive from a young age, but those that are non-aggressive are likely to become aggressive due to their environment.
MAO-A, 5HTT and violent crime
“Socio-psychological research underscores the relation between cognition, emotion, and aggression; negative affect such as fear and anxiety frequently precipitates, accentuates, and modulates aggressive behavior.”
In 2005 a group of scientists in Germany gathered to perform a study of violent criminal behavior. Their goal was to prove that a specific combination of genetics AND environment was necessary to create a human being capable of criminal violence, and this combination was the MAO-A, 5HTT gene and highly adverse parenting during childhood.
This study examined 184 adult Caucasian male volunteers referred for forensic assessment to the Institute of Forensic Psychiatry of the University of the Saarland by legal authorities for evaluation of legal responsibility or risk assessment. These men were categorized according to their propensity for violent behavior. All of the subjects of the study were mentally sound and showed no signs of mental disease or defect. All subjects had an extended history of either violent or non-violent behavior. The control for this study was in the form of 150 DNA samples taken from Caucasian men within the German population with no mental health issues or criminal history.
The methodology for this study was complex, involving personal interviews and DNA samples. The interviews were conducted by independent, blinded interviewers who questioned the subjects regarding their childhood, social status, income, etc. and rated the subjects’ answers according to optimal or adverse conditions. Venous DNA samples were taken from each of the subjects and examined for long and short alleles of the MAOA-uVNTR and the 5-HTTLPR genes.
Like the study itself, the results of the study are highly complex. The study showed a 74% correlation between a short MAO-A allele, an adverse upbringing and violent behavior. Subjects with a long MAO-A allele with an adverse childhood were less likely to be violent, as were those with a short MAO-A allele and an optimal upbringing. While the 5HTT gene showed no independent link to violence, those with this gene AND the short allele MAO-A AND an adverse childhood were more violent than those without the 5HTT gene. Thus, the team has proven their hypothesis that neither genetics nor environment alone makes a violent criminal; it takes both factors, working together to make a human monster.
Comparative Analysis and Conclusion
Both of these studies covered topics regarding the causes of adverse behavior in an individual. Both studies limited their scope to Caucasian individuals and relied heavily on input from their subjects regarding their individual upbringing. While both of these studies prove that a combination nature AND nurture are required to affect things like criminal behavior and psychopathic behavior, the studies were very different.
While the TCHAD study relied almost solely on input from their subjects and their parents, the MAO-A study included the element of DNA testing and a specific gene that is involved in the adverse behavior they were studying. While both studies reached similar conclusions, the use of DNA and specific genes in the MAO-A study lends more credibility and substance to their definitive results. The TCHAD study can only be as reliable as the subjects being tested are.
While both studies limited their subjects to Caucasian individuals, they chose very different groups to study. The TCHAD study subjects were all twins, and all were approximately the same age. This means that the study may have been skewed by environmental factors that may not apply to previous or subsequent groups. The MAO-A study limited their scope to adult males with a criminal history. Therefore, the results of this study may not apply to women or those with a mixed criminal history.
In conclusion, both studies revealed similar results. They used very different subjects and different methodologies, but reached similar conclusions. When these studies are looked at as part of the bigger question of Nature vs. Nurture, they successfully prove that the largest point of controversy surrounding this subject is true. Some things are not as simple as nature OR nurture, they require a combination of the 2 to make a substantial difference in the manifestation of a particular behavior.
Knowlton, L. (2005, July 1). Nature Versus Nurture: How Is Child Psychopathology Developed? Psychiatric Times , 1-5.
Paul Lichtenstein, C. T. (2007). The Swedish Twin study of CHild and Adolescent Development: The TCHAD-Study. Twin Research & Human Genetics , 67-73.
Richman, J. (2007, October 25). `Identical Strangers`Explore Nature vs. Nurture.
Samuel Wood, E. W. (2008). The World of Psychology. Boston: Pearson.