Omri Tal writes on the history of The Gloomy Prospect. Apparently, the term originally referred to non-shared environmental effects (see Random microworlds), before Turkheimer applied it to genetic causation.
Personally, I'm an optimist -- I believe in Pessimism of the Intellect but Optimism of the Will :-)
Just to point out with regard to your recent interesting post.
1. The Gloomy Prospect is a term originally by Plomin and Daniels (1987). As Turkheimer (2000) notes:
Plomin and Daniels (1987) almost identified the answer to this question, but dismissed it as too pessimistic:
"One gloomy prospect is that the salient environment might be unsystematic, idiosyncratic, or serendipitous events such as accidents, illnesses, or other traumas . . . Such capricious events, however, are likely to prove a dead end for research. More interesting heuristically are possible systematic sources of differences between families. (p. 8)"
The gloomy prospect is true. 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…
-- But indeed, it is Turkheimer's paper that has made the term famous.
2. The Gloomy Prospect is predominately about the unsystematic 'nonshared environment', rather than about missing heritability. In the section you quote, he extends this notion to include unknown genetic factors, but it's not the "classic" use ;)
Two interesting papers by Omri, at his web page:
Tal O, 2013. Two Complementary Perspectives on Inter-Individual Genetic Distance. BioSystems. Volume 111, Issue 1, Pages 18–36
Tal O, 2012. Towards an Information-Theoretic Approach to Population Structure. Proceedings of Turing-100: The Alan Turing Centenary. p353-369