In dit document worden de tweeling studies en adoptie studies beschreven. Er wordt een kort overzicht gegeven van wat het inhoudt. verder wordt er ook aangegeven met welke factoren er wordt onderzocht en hoe genen en omgeving af te leiden zijn van tweeling en adoptie studies. In dit document zit ee...
Behavioral genetics is the research field in which individual differences are explained by the
effects of Genes and Environment.
Two often used research designs are twin research and adoption research. In twin research,
the resemblance of MZ-twins is compared with DZ-twins. If identical twins are more similar
than fraternal twins, there is an influence of genes on behavior. If MZ-twins do not score
exactly the same, there is also an influence of the unique environment. Because twins and
family members grow up in the same family, especially during childhood, there is often the
influence of the common environment.
Familial outcomes are used in these types of studies, which you can express with
correlation, relative risk or discordance and concordance.
There are two options in adoption research. In the first method, also called the adoptee
study method, the starting point is the mother. In the second method, also called the adoptee
family method you look at the offspring.
Twin and adoption studies both have advantages and disadvantages. Advantages of the
twin research are that many twins are born and that there is no selection. Twins are born in
all layers of the population. D
isadvantages of twin research are that twins are often born with
low birth weight.
In adoption research there is selective placement, because adoptive families are often
bound to high requirements. The group is also small and often not representative. Adopted
children are also often born into less good circumstances. For example, research from the
Early Growth and Development Study shows that the average income of the biological
parents is lower than the average income of the adoptive parents. Also, their level of
education is lower and they are more frequently single. There is more open adoption
nowadays. This means that the contact of the biological parents with the child is maintained.
The advantage of adoption research is that it’s very powerful to separate the influence of
Genes and Environment.
Over the past years, more twins were born in the Netherlands and this is in particular a
consequence of the age of the mother. Because women become older the chance of getting
a DZ-twin increases. Identical twins are produced by one fertilized egg, while in case of
fraternal twins there is a double ovulation in the mother. Twins that are still partially together
are called siamese twins. It is essential to know the zygosity in twin research. This can be
determined by DNA or Questionnaires.
An assumption of the twin model is the Equal Environment Assumption. This assumption
implies that the environmental influences of identical twins is not more equal than the
environmental influences of fraternal twins. Results from twin research could be generalized
to the general population.
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