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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)?

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    $\begingroup$ Welcome. Please don't make questions too long; ask one question at a time, feel free to ask multiple questions in separate posts. As you post has an upvoted answer, I wouldn't make gross changes anymore here, but just for future guidance. Thanks. $\endgroup$ – AliceD Sep 28 '18 at 8:01
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Actually, you are misunderstanding twin studies when it comes to shared vs. non-shared environments. These are defined only in terms of outcomes:

The environmental influences on personality are divided into two main types in the behavior genetic literature: “shared” versus “nonshared” environment. In typical behavior genetic studies, actual measures of environmental factors are not obtained, but influences are inferred from the outcome pattern of observed similarities between subjects. Plomin, DeFries, et al. (2001; pp. 378–379; p. 300) define shared environment as “environmental factors responsible for resemblance between family members” or “family resemblance not explained by genetics”, and nonshared environment as “environmental influences that contribute to differences between family members” or “variance not explained by genetics or by shared family environment”. [...]

It is important to note that the distinction between shared and nonshared environment is made solely in terms of outcome, i.e., if an environment has the effect of making siblings more similar, it is defined as a shared environment, and if an environment has the effect of making siblings more different, it is defined as a nonshared environment.

But your confusion is common enough

Not only textbook authors, but even reports from behavior genetic studies oscillate between the outcome-based shared/nonshared distinction, and the causal-event-based variety. A recent description of the nature of “shared environment” is given by Reiss et al. (2000) in their book from the well known NEAD (Nonshared Environment and Adolescent Development) twin and sibling study, in explaining the results of minimal influence of “shared environment” (p. 68): “This analysis tells us that the major environmental influences on adolescents’ proneness to anxiety must be different for sibs in the same family. This rules out a number of influences, such as the family social class or the level of parents’ anxiety, all of which are shared by siblings in the same family.” According to the definition of “shared environment” this interpretation is incorrect when it comes to siblings living in their original family, since two siblings perfectly well can have different responses to a parent’s anxiety. Such gene-environment interaction would be accounted as a nonshared environmental effect in many commonly used behavior genetic models, in which nonshared environment incorporates interaction effects.

With this in mind however

The results of many heritability studies of the Big Five personality dimensions show that most of the environmental influence is attributable to nonshared environment. [As defined in the first quote].

Also note that since (according to the review of Turkheimer and Waldron) Plomin's group work was seminal, it makes sense to refer to defer to their definitions.

In what may have been the most influential article ever written in the field of developmental behavior genetics, Plomin and Daniels (1987) reviewed evidence that a substantial portion of the variability in behavioral outcomes could not be explained by the additive effects of genotype or the environmental influences of families. They suggested that this residual term, which they called the nonshared environment, had been neglected by environmentally oriented researchers who assumed that the most important mechanisms of environmental action involved familial variables, like socioeconomic status and parenting styles, that are shared by siblings raised in the same home and serve to make siblings more similar to each other. Indeed, Plomin and Daniels argued, once genetic relatedness has been taken into account, siblings seem to be hardly more similar than children chosen at random from the population. An important indicator of the influence of Plomin and Daniels' (1987) article is that an entire field of empirical research was generated in an attempt to answer the question posed in its title: Why are children in the same family so different? The content of this research was strongly influenced by Plomin and Daniels, building on earlier theoretical work by Rowe and Plomin (1981), who suggested that the causes of outcome differences among siblings were to be found in differences in the environments they experienced.

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  • $\begingroup$ Thanks! That's interesting. So would it be fair to summarize what is known on nature vs. nurture by "the only effect of the environment is in making siblings more different; basically never more similar"? Parents contribute only noise, without any signal? :) $\endgroup$ – Borbei Sep 27 '18 at 21:32
  • $\begingroup$ @Borbei, parents are nurture, but if under different nurture there is a similarity between either twins or even non-twin siblings, it is reasonable to think it's an effect of nature. Or vice versa, when nurture is the same, and there is a similarity between non-related people, it is more likely to be an effect of nurture. But that all works only for larger selections. Two persons are not enough, for sure. $\endgroup$ – rus9384 Sep 28 '18 at 14:19
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    $\begingroup$ One side note: the degree of heritability is itself a function of environment. The classic example is height: in times of plenty, 100% of variability in height is inherited, in times of famine, almost 0% is. Heritability is not a property of the trait alone. $\endgroup$ – steveLangsford Sep 28 '18 at 14:27

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