Explain how (a) twin studies, and (b) adoption studies are used to determine the influence of heredity on a trait and discuss a potential flaw of each type of study

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A good answer will be similar to the following:
+ Twin studies compare identical and fraternal twins to determine the influence of heredity. Identical or monozygotic twins come from a single fertilized egg that splits in two, and they have the same genes. Fraternal or dizygotic twins come from two separate eggs fertilized by two separate sperm and share, on average, about half their genes — just like regular siblings. In a twin study, if identical twins are more alike than fraternal twins on a particular trait or behavior, it suggests that heredity influences that trait or behavior. Potential flaw: Parents and other people may treat identical twins more similarly than they treat fraternal twins. This would make identical twins more similar than fraternal twins in their experiences, as well as in their genes.
+ In adoption studies, adopted children are compared to their adoptive parents and their biological parents. Adoptive parents have provided the child's environment. Biological parents provided the child's genes. If children are more similar to their biological parents than to their adoptive parents on a particular trait or behavior, it suggests that genes influence that trait or behavior. Potential flaw: Adoption agencies may try to place children in homes like those of their biological parents. This can bias adoption studies because biological and adoptive parents end up being similar.