Psychology - AS Level Twin and adoption studies

There are two types of twins:

• Monozygotic - identical
• Dizygotic - non identical

These will be referred to as MZ and DZ twins.

Why are twins used in psychology?
Twins are used to see if behaviours have a genetic or enviromental cause, as we can easily spilt them to determine what the cause of their behaviour is. It is useful that they have the same genetic make-up and they both at least shared one envirnoment - prenatal.

Concordance rate
A concordance rate is calculated. It sees what the percentage or likelyhood is that if one twin has a characteristic, the other will also have it.
These concordance rates can be expressed as a percentage (%) or as a decimal point (0-1). e.g. 50% or 0.50

We can compare MZ's with DZ's to see which pair is most similar or we can compare MZ twins reared apart.
If MZ twins are more more similar than DZ twins this indicates the role of genes as MZ's share more genetic material. If MZ twins reared apart are similar this also suggests genes are important as they do not share environments.

• Twin studies look at MZ's reared apart lack generalisability because only a small number of twins are reared apart. This means the results of such studies cannot be applied to other people. 
• Twin studies also lack generalisability because participants are gathered using volunteer sampling. Only certain types of people come forward giving a biased sample. The results cannot be applied to people who do not volunteer. 
• Twin studies lack internal validity because twins share genes as well as environment. Even MZ's reared apart share pre-natal environments. This means we do not know if they are more similar because of their genes or their shared environments (cause and effect). 
•   Twin studies also lack internal validity because parents tend to do treat MZ's as the same person which they don't tend to do with DZ's. This means they share a more similar environment as well as more genetic material which could explain higher concordance rates. 
• Twin studies where MZ's are reared apart lack internal validity because adoption agencies try and place twins in similar families meaning they still share genes and environment so cause and effect cannot be established. 

• Twin studies are one of the only ways we can begin to assess the contributes of genes and environments to our behaviour. They attempt to isolate genes and environment so that cause and effect can be established. 

MZ twins 49%
DZ twins 9%

Explain the contribution of genetics and environment to the development of Schizophrenia (SX) - 
The fact that the concordance rates are higher for MZ twins (49%) supports the roles of genes in MZ twins share more genetic material than DZ twins. However, the concordance rate for MZ twins should be 100% because they share 100% of their genetic material. Also the concordance rates for DZ twins should be 50% because they share 50% of their genetic material. The fact that the concordance rates are much lower, indicates the role of environment in SX.