What are genes?
The entire genetic material of an organism with a nucleus in its cells is distributed over several DNA molecules that are tightly packed with protecting proteins. These structures are called chromosomes. There are areas in the chromosomes that do not contain genes. These are important for controlling the expression of the genes and other functions.
How do genes work?
A gene contains the information necessary to build a specific protein. This information is stored in the sequence of the building blocks of the DNA molecule the four bases Adenosine, Cytosine, Guanine, and Thymine or short A, C, G, and T. To build a protein according to the instructions in the gene, a RNA-copy of the DNA is made and transported form the cell’s nucleus to the main cell body. Specific enzymes can read the information on the RNA and translate it into the correct sequence of amino acids, the building blocks of proteins. There are about 20 different amino acids available and the information for one amino acid is contained in a sequence of three bases on the DNA.
How do genes impact behavior and personality?
How can a sequence of bases in the DNA that encodes a sequence of amino acids influence possibly our behavior and personality? The step from the working of a single gene to such complex features as personality and behavior is huge. However, if genetic information can determine how such complicated features like the eye of vertebrate develop, it is not too fantastic to imagine that it can also influence personality and behavior.
How exactly this happens is to date not fully understood. However, there are correlations between the physical development of the brain dependent on e.g. gender that have a strong influence on behavior. The amount of expression of certain messenger molecules called neurotransmitters in the brain can also have a large impact on behavior and can even lead to such diseases as depression or schizophrenia.
The expression of neurotransmitters and also of receptors for them is controlled by the product of certain genes and it is hypothesized that genetic polymorphisms in the expression levels can predispose people for alcohol and drug addictions and other disorders. Personality traits are even more complex than predispositions for mental disorders and we are still at the beginning of the understanding how genes can influence these.
How can the heritability of personality traits and behavior be measured?
How do we know at all that genes have an impact on personality and behavior i.e. how is heritability of personality and behavior measured? The contribution of genetics to behavior and personality in laboratory animal can be measured easily, since in this setting the breeding (genetics) and the environment are easily controlled.
For humans studies this is not possible. Researchers therefore resort to large and complicated family studies. Twin studies are very popular in this context. Some studies e.g. compare identical twin pairs that have been brought up separated form each other to the general population. Identical twins share all their genes, but the environment is different in this context. Other studies compare identical twins to fraternal twins who all share the same family environment, but the fraternal twins share only half of their genes. Looking at adoptive siblings, who share the environment, but no genes compared to biological siblings who share half of their genes and the environment, can also be helpful.
In meta-analysis of studies that looked at the Eysenck Personality Questionnaire (EPQ) dimensions (Neuroticism, Extraversion, and Psychoticism) it was found that the results for identical twins had a correlation of 0.468. A correlation of 1 would mean that the answers were 100% identical. Fraternal twins had only a correlation of 0.166, similar to other first-degree relative who had a correlation of 0.150. Second-degree relatives had a correlation of 0.073 and adoptive family members had only 0.030. This shows that there is a string genetic component to these personality traits, but environmental influences do also exists, otherwise the correlation between identical twin should be very close to 1.
What is the nature versus nurture debate?
“Nature vs. nurture” is the tag line for the old question whether behavior and personality traits are most influenced by genetics (“nature”), or by the environment (“nurture”). Most scientists nowadays agree that to put this question this way is overly simplistic. Genetics might e.g. lead a certain person to seek a certain environment that than in turn influences the behavior and personality of this person. On the other hand, environmental stimuli can influence the expression levels of certain genes, so that the environment can contribute to the influence genetics has on the individual. This principle is best understood for genetic diseases that depending on the environment become apparent or not despite the fact that the individual has the gene for this disease. But it also is true for personality traits. It is therefore not helpful to think that there is a strict segregation between environmental influences and genetics.