In the following I have 4 questions which I think are too interleaved to separate into 4 different posts. I hope it's not a duplicate, but I didn't find here similar posts.
My understanding (as a layman, based on few non-professional books and some googling) is that the current "state of the art" regrading the age-old question in the title, is essentially as following:
For pretty much any reliable measurement researchers could come up with (in relation to cognitive capabilities, personality traits, interests, confidence, success, etc), 100% of the variance could be attributed to heredity and nonshared-environment (typically in approximately 50-50 split between the two), and effectively 0% of the variance is attributed to shared-environment.
My impression is that the research is vast, the data is ample, and the conclusions are overwhelmingly consistent and accepted.
My first intermediate question: is the above description correct?
Now, that trichotomy, as I understand it, goes like this: by "heredity" people refer to anything identical twins that grow separately share (so mainly genetic and epigenetic factors), by "nonshared-environment" people refer to anything identical twins that grow together do not share (specific social interactions, different life experience, etc), and by "shared environment" people refer to the things non-sibling that grow together share (same parents and family, same home, same neighborhood, same school, same lifestyle etc).
My second intermediate question: is this description correct?
If so, it basically means that a pair of identical twins that grew up together, is indistinguishable from a pair of identical twins that grew up separately, and that a pair of any two random people is indistinguishable from any two non-sibling that grew up in the same home with the same parents.
In shorts, sharing home does not make people more similar, with respect to most reliably measurable properties.
My third intermediate question: are there any notable exceptions? Known traits whose variance is meaningfully explained by the shared-environment?
My forth and last question: Is the socioeconomic status of the family is part of the shared-environment? It seems as it should be (if my description above is not too-wrong), but then it implies that it has no effect on anything other than the future socioeconomic status of the children. Is this really case?
Do things like the neighborhood and schools quality have no intrinsic effect (e.g. on intelligence, inclination to violence, occupational aptitude, religiousness, etc)?