A twin study is a psychological study which focuses specifically on twins. These studies are used to explore the science of behavioral genetics, and several psychologists actually specialize in twin studies. The validity of twin studies is questioned by some researchers, especially when psychologists place heavy emphasis on the results of a twin study. Others argue that they are very valuable, even when used only as a starting point for further scientific inquiry.
Twins are unique because they share genetic material. Identical twins are actually genetically identical, while fraternal twins share half of their genetic material. This allows people who are interested in studying the role of genes in personality development to control the circumstances of their studies a bit more. Since psychological studies on humans can be ethically ambiguous, a twin study is one way to examine the interplay of genetics and environment when examining personality development without crossing ethical boundaries.
In most cases, a twin study follows twins which grow up in the same household. Ideally, the study should compare other types of siblings growing up in the same household, as a basis for comparison. When twins demonstrate more similarities than conventional siblings, these similarities are assumed to be genetic. Other twin studies focus on twins who have endured separation at birth, such as twins who have been adopted out, in a further effort to look at the role of environment on personality.
Critics of twin studies point out that a number of factors influence personality, and that even established genetic personality traits are extremely complex. These traits involve the interactions of multiple genes, and it may not be entirely fair to evaluate personality on the basis of a study which may be influenced by other factors. For example, some people may be genetically inclined to carry twins, which might skew such a study. In addition, a twin study assumes that twins are treated equally, making their environments comparable, and this may not always be the case.
As is often the case with scientific research, twin studies are a topic of debate. It is important to carefully analyze the results of any psychological study, since human psychology is very complex. A twin study may make the mistake of turning correlation into causation, for example, or it may place emphasis on outdated research methods. Even with obvious flaws, however, twin studies are certainly interesting, and the concept of the twin study also laid the foundations for the field of behavioral genetics.