Twin studies versus sibling studies explores genetic versus environmental influences.
One way to isolate genetic factors is to try to control for the environmental factors by studying twins and same age siblings to see what degree of the character or behavior is genetic versus environmental.
It is important to compare both dizygotic DZ twins with monozygotic MZ twins ,when possible, since they both experience a twinned environment during embryogenesis and after birth. Comparing MZ to DZ to siblings allows the genetic differences to stand out from environmental influences.
Twin studies of autoimmune diseases indicate there are environmental triggers that influence whether the genetic condition .
"Twin Study Spotlights Alzheimer's Genetic Link'
http://pn.psychiatryonline.org/content/41/7/32.fullMS twin study shows environment plays key role
http://www.bionews.org.uk/page_59451.asp"A twin study of cognitive function in chronic fatigue syndrome: the effects of sudden illness onset."
http://www.ncbi.nlm.nih.gov/pubmed/17605583http://www.bionews.org.uk/page_59451.asp