Twin Studies
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes and disorders. Twin research is considered a key tool in behavioural genetics and in related fields, from biology to psychology.
Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: “identical” or monozygotic (MZ) twins share essentially 100% of their genes, which means that most differences between the twins (such as height, intelligence, depression, etc.) are due to experiences that one twin has but not the other twin. “Fraternal” or dizygotic (DZ) twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family.
The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implies that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behaviour.
Nature and Nurture:
Nature versus nurture is a long-standing debate in biology and society about the balance between two competing factors which determine fate: genetics (nature) and environment (nurture). Nature is what people think of as pre-wiring and is influenced by genetic inheritance and other biological factors. Nurture is generally taken as the influence of external factors after conception e.g. the product of exposure, experience and learning on an individual.
The view that humans acquire all or almost all their behavioural traits from “nurture” was termed tabula rasa (‘blank tablet, slate’ by John Locke in 1690. A blank slate view in human developmental psychology, which assumes that human behavioural traits develop almost exclusively from environmental influences. The debate between ‘blank-slate’ denial of the influence of heritability, and the view admitting both environmental and heritable traits, has often been cast in terms of nature versus nurture.
In ecology and behavioural genetics, researchers think nurture has an essential influence on nature. Similarly, in other fields, the dividing line between an inherited and an acquired trait becomes unclear, as in fetal development.
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