Follow-Up: That Turkheimer quote broken down

A friend has told me that he would like to have this Eric Turkheimer quote which came up in my last post broken down, as it's pretty dense:

Nonshared environmental variability predominates not because of the systematic effects of environmental events that are not shared among siblings, but rather because of the unsystematic effects of all environmental events, compounded by the equally unsystematic processes that expose us to environmental events in the first place.
(From this article.)

I'll give it a go; I might completely misjudge where the difficulties lie and end up breaking it down in a way that's completely unhelpful :D I will rely on readers' feedback and rewrite this post as much as necessary. Should this task prove difficult to me, then I can probably learn a lot from doing this.

Nonshared environmental variability predominates

The thing of "X-type variability predominates" can somewhat grossly be rephrased as "X has the largest effect". Here we're speaking about effects on psychological/behavioural outcomes. Turkheimer is basically saying that the specific differences between adults are predominantly explained by differences they have experienced in their "nonshared environment". For example: people differ in what language they speak; and as it turns out, the language you grow up to speak and the accent you grow up to speak it in depends to a much larger extent on the peer groups you grew up around (nonshared environment) than on what language your parents spoke (shared environment).

As a quick reminder: "shared environment" refers to the environment shared by siblings (in the sense of "children raised by the same parents", not in the sense of "biological siblings"); more specifically, you would look at the ways in which people differ from each other and check whether "the fact that those people were raised by the same parents" has made them any more similar (on average) than the data would lead you to otherwise expect. "Nonshared environment" refers to everything that is neither inherited nor a factor of "shared environment".

So is Turkheimer saying that "nonshared environment" is more important than heredity? No, because in the context of this quote, he is only comparing the two kinds of environment with each other; in other words, he means that "nonshared environmental variability predominates over shared environmental variability".
(As to whether he thinks nonshared environment is more important than factors of heredity, I think I know his position well enough to answer this pretty confidently: He would say that, in the case of most psychological outcomes, all that behavioural genetics can really tell us is that nonshared environment and heredity are both substantial contributors whereas shared environment isn't, and that attempts to put a precise number on the relative effects of the two important contributors are not to be taken particularly seriously because the methods just aren't precise enough. And for the record, I agree with that.)

To be clear, what he's saying is more precise than "X has the largest effect". The statistical method used in these studies is analysis of variance, in which you determine the "amount of overlap" between variation among individuals in the dependent variable with variation among those individuals in the independent variables (=factors). Whether this means anything to you or not, just appreciate that we're talking correlations here, and that there is some risk of drawing invalid conclusions about causation from it.
Remember, though, that while correlation does not imply causation, absence of correlation DOES imply absence of causation, which is why we can be so confident that the "shared environment" (or "who raised you") is not an important predictive factor.

Onward:

Nonshared environmental variability predominates not because of the systematic effects of environmental events that are not shared among siblings, but rather because of the unsystematic effects of all environmental events

Ok, the statistical methods used in those studies allow us to rather crudely distinguish between three "sources of variability", or, basically, three broadly defined factors: heredity (who you got your genes from), shared environment (who raised you), and nonshared environment (absolutely everything else, from your peer groups to illnesses to the fluctuations in the electrochemical environment of your synapses as they were forming).
And, once again, the results are very consistently as follows: heredity very important, shared environment unimportant, nonshared environment very important.
The initial assumption, or at least the initial hope of researchers like Robert Plomin was that the relevant aspects of the nonshared environment would soon be identified, allowing for systematic interventions to reduce the incidence of mental disorders and perhaps increase the incidence of desirable outcomes such as high IQ.
Turkheimer was basically saying that, with all that Plomin-inspired studies had taught us, we might as well drop the distinction between nonshared and shared environment and just say that: 1. who people got their genes from is important, and 2. their individual environment is also important, but that whatever specific environmental factor you may extract will turn out to have, at best, a very small systematic effect - and "who raised you" just happens to be one of those many environmental factors which do not allow us to make any useful systematic predictions about most psychological outcomes.

Nearly done now:

compounded by the equally unsystematic processes that expose us to environmental events in the first place.

In other words: not only can the effects of what people encounter in their environment not be predicted (with sufficient accuracy), but what people will encounter in their environment cannot be predicted either.

I hope that'll do :D