What are the problems using twins to determine whether genetic ...

[camila]

Disorders?

I have been given this as a homework assignment question and I genuinely have no clue why this would be a problem. Any help would be greatly appreciated!

[Sandstorm]

The first thing to consider is whether you are looking at monozygotic (identical) twins or dizygotic (fraternal) twins. The other important fact that must be considered is that genes alone are not usually causative for a disorder which follows the standard rule of phenotype = genotype + environment. In practical terms this means that unless a particular gene effect is completely "penetrant", that is it always causes an effect when it is inherited, twin studies could assign no effect to a particular gene that is only partly penetrant. In the cause of psychological and neuropsychiatric diseases, they are usually not monogenic but the result of numerous genes and environmental influences.

[curlz]

The best scientific "twin studies" are those in which identical twins had been separated at birth and raised by completely different families, and had no interaction with one another.

That way, they have exactly the same genes, but completely separate environment.