3. Then you need twin for every medical research.......
Nope. You really don't. Most studies like this you could test things first on lab animals to establish the principle of the disorder and then move to clinical testing after that concept was proven. Also, the rest of a human body is fairly well studied with a huge number of clinical cases to back up research.
You'd need twins for
this study because it's researching a correlation between brain development (which really is mainly in the theoretical sciences) and an active choice. Neuroscience is just at the beginning of research (comparatively) and there's still a lot that isn't known about how it works. We do know that the brain makes choices, though. If a "decision making" center change in the brain in a still developing brain shows a correlation with marijuana use, we don't know if this is because marijuana changes the development of the brain or because people who have this anomaly in their brain are more likely to smoke marijuana and also indulge in a handful of other risky behaviors.
And since the sample they were studying consisted of a group of people who's brain would have been undergoing changes
anyways, it's important to have a clonal population to verify what the cause of these changes are.
It's similar to when they used to say marijuana could cause psychotic breaks. No, it can't. However, for people with a history of schizophrenia in their family, if they smoke marijuana, will typically suffer their first episode while under the influence due to a number of other factors. But it's something that would have happened regardless of drug use.
