Genetics and Twins

Twin Studies
This studies and observations are based on genetic information of twins. These studies are mainly used to determine the impact of the environment and genetics on humans.

Adoption Studies
This studies observe the nature versus nurture impact on the development. It is focused on children with disorder. Its adoptive parents,however dont have such a disorter, only their biological parent does. So that we can observe how the disorder develops and whether it is caused and impacted only by gengetics or also the enviroment.
  
(MISTRA) Minnesota Study of Twins Reared Apart
The MISTRA is considered one of the most important studies done in the field of psychology in the last 50 years. It began in 1979, at that time it was widely believed that intelligence and personality were molded by the environment. It ended 20 years later and proved that who we are is in a huge aspect determined by the genes. There are two types of twins; the monozygotic (MZA) when an egg splits into two in the womb and the twins have the same genes and dizygotic (DZA), when two eggs are fertilized and the twins have about 50% same DNA.
In this study they observed 81 pairs of MZA and 56 pairs of DZA twins, the DZA twins were used as controls so that they could rule out the environment as a reason why are the MZA twins similiar. They used twins who were apart from majority of their lives, separeted before the age of four. Many of these twins lived in different countries. Most of the twins were separeted because they were illegitimate. In many cases they didnt even know that they had a twin. Some of them started to look for their birth parents and found out they had a twin, or they learned of each other because of mistaken identity(this happened mostly with MZA twins).
The study says that intelligence levels of MZ twins are very similar and that testing them is like  "testing the same person twice".
The first twins that were examined met at the age of 39 and were males. They both worked in the same departments, in law entforcment and later on as firefighters. And also both of them loved mathin school and hated spelling, they even shared favorite vacation spot. As they studies more MZA twins they found out even though they were not aware of the existence of each other, they are similiar in everyway, even naming their kids. The MZA twins even arrange their hands and legs in the same way, the DZA do not.
This study shows that personality is about 50% inherited in genes and the environment that the person makes for himselt, or the ''non-shared'' environment.The household, the ''shared'' environmnent, has some impact but it was shown that twins reared apart and twins who frew up together have very similiar yet no indentical personalities. Today parents still think that how they raise their kids will determine their personality.
 MISTRA found out that the heredity is very high for drug and alcohol use. The intensity of religion is also inherited, however the religion is tought by the environment you live in. Also the morning vs evening person is inherited. 
This study also shows that the sexual orientation is inherited, mostly by men, women seem to be more affected by the environment. The DZA twins of opposite sex were often flirtatious with each other. ''This is not surprising; genetic similarity theory predicts it. Biological fathers/daughters and mothers/sons can be sexually attracted if they have not lived together. In the case of reunited male MZA homosexual twins, attraction can lead to sexual relations.'' (Thomas Jackson, American Renaissance, July 27, 2012)
 This study could have been unethical in some aspects. One of this aspects is telling a person that he has a twin. They did not know of each others existence and this might have contributed to future worrying about a genetic disease that the other twin has and the other does not.