Despite our general physical similarity as a species, human personality traits vary greatly among groups and even individuals. The diversity of our behavior has influenced psychologists from different perspectives to more thoroughly examine what makes an individual act the way he/she does. This search has been mostly expressed through an ongoing debate that has been around at least since the days of Ancient Greek philosophers such as Plato and Aristotle. Is human behavior predetermined primarily by our biology or is it developed through our experiences and the environment in which we are raised?
Nature vs. nurture: the famous battle that has plagued studies of human behavior for centuries. “Nature” refers to our biology - our physical and genetic makeup. “Nurture” refers to our environment and experiences - the external world we live in. Which is the primary influence on our personalities? Over the years, this has proven to be a topic of great interest, with many supporters on either side of the issue. Indeed, some of the greatest psychological discoveries of modern times may have resulted from research conducted in the name of the nature/nurture debate.
Most psychologists today, as well as most biologists and philosophers, agree that both nature and nurture are important in the development of human behavior. Furthermore, it is generally accepted that the effects of nature and nurture intertwine. After all, as recent findings in neurology display, every psychological event is linked to a simultaneous biological event. Our behavior, physiology, and environment are all connected, each constantly influencing the others.
By examining the basics of each side of the issue, we can find further examples of how nature and nurture work separately as well as together in the development and expression of human behavior.
Nature
Genes
Each cell of the human body contains 23 pairs of chromosomes - 46 in total, each consisting of thousands of genes. We receive half of our genes from our mother, the other half from our father. Genes, self-replicating segments of complex DNA molecules, are the biochemical units of heredity. Essentially, they are the blueprints for our physical and behavioral development. Genetically, humans are 99.9% similar. The variation that still occurs in certain parts of our DNA, however, defines our individual differences. In fact, with the exception of identical twins, each person has a unique genetic profile.
Through the extensive work of the Human Genome Project, scientists have begun to identify genes that influence certain diseases and disabilities. For example, Huntington's disease is considered a genetic illness, and there is a 99.9% correlation between having the disease and the gene associated with it. The same correlation applies for those that do not have either.
The genetic foundations of behavior, on the other hand, have proven much more difficult to identify. It seems that physical traits, such as hair and eye color, are far more hereditary than personality traits. We often notice behavioral similarities between parents and offspring and sometimes attribute them to inheritance. However, researchers have yet to positively identify genes responsible for specific behaviors, and so studies of heredity in personality are not conclusive. You may believe that your mother or father passed their bad temper down to you, but you'd have a difficult time convincing a geneticist of that.
Another factor to consider is the fact that genes do not work in isolation. The concept of a singular gene causing a singular trait has been discredited through research. Our DNA acts in complex ways as genes receive signals from each other as well as from the environment. The influence of genes on traits, both physical and psychological, seems to depend greatly on particular environments and conditions.
Twin Studies
Some scientists attempt to discern between the effects of genes and environment by studying the lives of twins. Fraternal twins develop from two separate eggs and are no more similar genetically than any other brothers or sisters. Identical twins develop from a single fertilized egg that splits in two. Therefore, identical twins are 100% similar genetically. If our behavior is shaped by our genes, then identical twins should be more similar than fraternal twins.
In fact, studies over the years have concluded that, more or less, it is true: identical twins are more alike in personality traits, interests, preferences, and even talents than fraternal twins are. However, identical twins also report similarities in the way they are treated by others. One factor influencing this is social reaction to the same phenotype (physical appearance). Experience rears its head again. Are people that are treated the same prone to behave the same way?
To answer this question, studies have been done on separated twins. The conclusion is that identical twins raised by different families in different environments show more dissimilar personality traits than identical twins raised together. However, separated identical twins were still more alike than separated fraternal twins. The actual amount of influence environment has on these type of studies is debated by critics, but the research does imply that genes play a role in our behavioral development.
