100% of their genetic variation and all of their shared environment when reared together. Dizygotic twins (DZs) result from two eggs, fertilized by two sperm, and therefore share, on average, just 50% of their genetic variation (as do ordinary siblings), but also share 100% of their shared environmental influences when reared together. Accordingly, comparing the similarity of MZ and DZ twins yields information about the relative importance of genetic and environmental influences. To the extent that MZs are more alike than DZs, genetic influences are implicated. If DZs are just as similar as MZs, then shared environmental processes, such as those influences found in the shared family environment, shared peers, shared schools and neighborhoods, etc., must predominate. If MZs are not exactly identical (as they would be if an outcome were 100% genetically influenced), then unique environmental processes must play a role. These could include environmental influences that are unique to an individual, such as a particular life event, stressor, or other influence not shared with their co-twin, and/or environmental events that differentially effect the co-twins (Turkheimer and Waldron, 2000).
