People vary in a number of psychological characteristics. These differences are due to the different conditions of life as well as dissimilar genotypes, since people’s genotypes have different forms of genes. Psychogenetics studies correlative contribution of heredity in a variety of people to the psychological properties and behavior. To estimate the influence of heredity on human behavior, the scientists compare people with different degrees of genetic commonality (monozygotic twins, native and half-brothers and sisters, children and their adoptive or biological parents) (Slee, 2002).
To date, scientists found and decoded genes that are connected with the ability to read and to remember, aggression, antisocial behavior, depression, and susceptibility to frequent change of partners. If to believe all the researches on hereditary problems, which were released in the last twenty years, it is unlikely to be able to find at least one character trait and the ability of an individual for which any gene is not responsible. Charles Darwin’s cousin, Francis Galton was the first who began to investigate the influence of genes on human behavior (Halverson & Tirmizi, 2008).
Many genes exist in several forms, just as there are different forms of the gene that determines the eye color. Some genes have dozens of forms. Genotype of an individual has two copies of each gene, forms of which may be different or the same. One is inherited from father and the other one - from mother. The combination of forms of genes is unique for each human body. This uniqueness is the basis of genetic variety between people. The contribution of genetic differences in the diversity of people to the psychological properties represents an indicator that is called the “coefficient of heritability”. For example, for intelligence, the coefficient of heritability is at least 50%. This does not mean that 50% of intelligence that is given to a man by nature and the remaining 50% must be added through education, and then the intelligence will be 100 points. The coefficient of heritability is not related to a certain individual. It is calculated in order to understand the reason of differences of people from each other: if there is any difference because people have not the same genotype, or because they were taught differently (Sharma & Sharma, 2006).
If the coefficient of heritability of intelligence was close to 0%, then one would conclude that only education creates differences between people, and the use of the same educational and training methods for different children will always lead to the same results. High values of heritability indicate that even with the same education, children will be different from each other due to their genetic characteristics. The end result, however, is not determined by genes. It is known that children that were adopted by good families are close to their adoptive parents and may well exceed biological ones in terms of intellectual development. Scientists studied two groups of foster children. Conditions in foster homes were all equally good, and biological mothers of children varied in a level of intelligence. Biological mothers of children in the first group had a higher than average intelligence. Approximately half of children in this group showed higher than average intellectual ability, while the other half - the average. Biological mothers of children of the second group had a slightly reduced (but within the normal range) intelligence. Out of this group, 15% of children had the same low rate of intellect. The other children’s intellectual development level was equal to the average. Thus, under identical conditions in foster families, children’s intelligence, to some extent, depends on the intelligence of their biological mothers (Sharma & Sharma, 2006).
The above example may serve as an illustration to the significant differences between the concept of heritability of psychological traits and heritability of some physical features of a person, such as the eye color or skin. Even with a high level of heritability of psychological trait, genotype does not prejudge its final value. A certain genotype predisposes to a child development in certain environments. In some cases, the genotype specifies “limits” of the severity of a trait. Studies show that genes are responsible for 50-70% of the diversity of people in terms of intelligence and for 28-49% of the variation in the expression of five “universal”, the most important properties of an individual:
- Intellectual flexibility (Slee, 2002).
Most psychogenetic researches that were conducted directly on humans tend to study the influence of genes on antisocial behavior or addiction. The ability to antisocial behavior in a person is one of the most important factors that scare adopters in different countries. Many families that decided on adoption are afraid of raising a child who will still be a bandit, no matter how much to invest in his/her education. However, the studies of Swedish and American scientists found that being in a foster home with a high socio-economic status, children whose biological parents had a criminal record almost never subsequently violated the law. If the host family had a low socio-economic status, the risk of antisocial behavior in these children increased. That is the most favorable environment that can counteract genetic predisposition to violate the law. One cannot deny that heredity has a significant impact on the human physique. Such qualities as agility, high speed, and flexibility are often descended from generation to generation (Beenstock, 2012).
Meanwhile psychogenetics studying the interaction of factors of heredity and environment in the development of a person do not cease to repeat: genes provide only a predisposition to a particular character trait or ability, but what to do with it - to develop or ruin - depends on each person (Beenstock, 2012).