Every day we all observe that people differ from one another and behave distinctly in similar situations or behave similarly in different situations. While, for instance, some people apparently socialize easily, others keep a low profile.
But how can these obvious differences be explained? Are they a product of our dispositions and so to speak our genetic material? Or can they rather be explained by mutual and differing environmental conditions? Since its early beginnings psychological research is concerned with these questions and among other things hopes to find answers through twin research.
Methods of twin research
Twins represent something very special and precious for psychological research because the examination of psychologically relevant characteristics in a sample of twins gives interesting information in regard to the question why people differ from one another. Every human-being is the unique result of the interaction of innate (inherited) dispositions and environmental influences. The twin studies allow for an analysis of the impact of genetic and environmental factors in a unique manner.
These analyses are based on a comparison of the resemblance of identical and fraternal twins. We take advantage of the fact that identical twins possess exactly the same genetic disposition and also many of their environmental influences (e.g. family lifestyle). In contrast, on average fraternal twins only share about 50% of their genes. However, compared to normal siblings, fraternal twins - as well as identical twins - assumedly share more environmental factors with each other because they grow up in the family at the same time.
Thus, exactly this comparison between identical and fraternal twins enables us to estimate the impact of genes and the environment on the characteristic of interest. If, for instance, identical twins show more similarities in one characteristic than fraternal ones, this indicates that genetic factors contribute to the individual development of this characteristic. If, in contrast, identical and fraternal twins do not or barely differ from one another in their resemblance, this would indicate an insignificant impact of genetic factors. The latter case would indicate that the cause of the resemblance of the twins might rather be found in the realm of the environmental factors.
Further methods contain the comparisons of twins with other siblings, with adopted siblings, with parents and close relatives.
Nature and nurture – the notion of heritability
Some feel uncomfortable when they imagine that our genes are involved in different traits – be it cognitive abilities or motivation. Often the high heritability of a characteristic is falsely understood as the immutability of the characteristic. However, the notion “heritability” only represents a statistic indicating which percentage of individual differences within a population can be explained by genetic differences between the examined persons.
According to that a heritability of 70% would imply that 70% of the differences between persons can be explained by genetic differences. This becomes even clearer in an experiment of thought: If we assumed that a group of children grew up in the exact environment and made equal experiences, then 100% of the differences among the children could be traced back to their genetic differences. The question that should be answered at this point is: To which degree can the scope of a human characteristics or rather its variance be explained by genetic differences? At the same time the following question is implied: Which portion of the variance of human behavior cannot be explained by predisposition but instead by environmental influences?
If differences between individuals in, for instance, their intelligence are influenced 50% by hereditary factors, environmental factors must be equally important as genes for implementation of these differences. Our behavior is influenced by our genetic raw material as well as by our environment and its interaction. It does not succumb to any “genetic determination” at all. Nevertheless, the relative meaning of nature and nurture for different areas of behavior can differ and change during the course of our development. Our genes often determine the limits within which the environment can exert its influence. Here, the development of the ability to read of elementary school children can be used as an example. Our genes may determine how fast we can learn new words and comprehend relationships, but if this basic ability is not nurtured during childhood, we would hardly be able to understand or produce complicated texts as adolescents. Daily reading with parents, a good teacher at school, or friends who also enjoy reading can influence, change and foster our genetically determined linguistic potential.