Twin Studies:
Studies and observations based on genetic information of twins. These studies are mainly used to determine the impact of the environment and genetics on humans.
Adoption Studies:
Studies that observe the nature vs nurture impact on psychological development. The focus falls on children with disorder and whore biological parents have it, too, but adoptive parents do not. This helps to show how the psychological illness develop and whether they are caused and impacted only by genetics or also environment.
Minnesota Study of Twins Reared Apart (MISTRA)
MISTRA is a study done in times when it was widely believed that personalities are 100% molded by the environment. It lasted from 1979 and ended 20 years and it is considered one of the most important psychological studies for past 50 years.
There are different types of twins; monozygotic when an egg splits into two in the womb and the twins have the same genes and dizygotic, when two eggs are fertilized and the twins have about 50% same DNA. MISTRA observed 81
monozygotic twin pairs (MZ) and 56 dizygotic twin pairs (DZA)who were used as controls, to find out which traits are given by the genetics and which are determined by the environment. The result was shocking for the time period; genes have a huge impact on how we develop.
Candidates for research were twins separated before the age of four who lived apart more than 95% of their life. Many times these siblings lived in different states and acknowledged each other existence during the MISTRA research. Some, however, met before while searching for their biological parents or just by coincidence, but this variable was also used as a research and suggests that the quirky similarities between twins weren't modified by their acquaintances.
The studies say that intelligence levels of MZ twins are very similar and that testing them is like "testing the same person twice".
Big surprise were the personality findings where was assumed that personality is formed by the environment. Studies shown that personality is about 50% given by the genes and the environment that person makes for himself, or the "non-shared environment". Many parents think that how they raise their children determines who they will become, but they are wrong. The household, the "shared environment", has some impact but it was shown that twins reared apart and twins who grow up together have very similar yet not identical personalities.
Among traits that are determined mostly by genes are hand coordination becoming better with practice, headache frequencies, dependency on alcohol, intensity of religion (religion is determined by the environment and usually taken over from parents), morning vs evening person, likely hood of gum disease (even with different diets) and spelling abilities.
As far as sexual orientation, men are more more influenced by the genes, while women are more to be influenced by the environment, for example they are more likely to establish relations with women when they are not attractive to men.
Children who grow up together develop subconscious anti-incest mechanism. It wasn't surprising then that the DZA different sex twins were often flirtatious which each other when they met.
Ethical Issues
Most twins separated at birth were not even aware they had a twin. For some, it might have been a traumatic experience to find out they are adopted. Questions like "who are my real parents?" or "Why didn't they keep me?" appeared in their minds. Also, if one twin suffers from some genetic disorder, the other twin is most likely but not for 100% going to have the same condition. This other twin is very likely to spend their life thinking about getting the disease and worrying and in the end may not even get it.
Pros
The science behind twin and personality was cleared, not completely, but significantly. It is known now that personality is very much affected by the genes but not entirely. It is still known to what extent but it leads to more knowledge and maybe even as a semi-solid bases for new studies. If scientist will be able to identify which gene exactly does what or what environmental causes trigger of certain habits and how they can prevent someone, for example from alcoholism, it would significantly improve one's life.
Cons
"Labeling something doesn't explain the process by which it happens" says Evan Balaban, a scientist who transports brain cells from quails to chickens so they sing like quails. Also, there are many genes which interact in many different ways that at this rate it will take a lot of time for science to figure out how it works. Also, if everything was proven to be genetic, the society would attack the environmental causes that would eliminate some people.
Sources:
http://www.reference.md/files/D018/mD018486.html
http://aje.oxfordjournals.org/content/155/7/585.full#sec-11
http://www.psychology-lexicon.com/cms/glossary/glossary-a/1582-adoption-study.html
http://www.amren.com/features/2012/07/science-versus-ideology/
Nature and Nurture: When It Comes to Twins, Sometimes It's Hard To Tell the Twin Apart by Arthur Allen; Washington Post (Sunday, Jan 11, 1998)
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