Abstract: The importance of non-shared environment lay hidden within quantitative genetic studies since they began nearly a century ago. Quantitative genetic methods, such as twin and adoption methods, were designed to tease apart nature and nurture in order to explain family resemblance. For nearly all complex phenotypes, it has emerged that the answer to the question of the origins of family resemblance is nature—things run in families primarily for genetic reasons. However, the best available evidence for the importance of environmental influence comes from this same quantitative genetic research because genetic influence never explains all of the variance for complex phenotypes, and the remaining variance must be ascribed to environmental influences.
Yet it took many decades for the full meaning of these findings to emerge. If genetics explains why siblings growing up in the same family are similar, but the environment is important, then it must be the case that the salient environmental effects do not make siblings similar. That is, they are not shared by children growing up in the same family—they must be ‘non-shared’. This implication about non-shared environmental import lay fallow in the field of quantitative genetics because the field’s attention was then firmly on the nature–nurture debate. ‘Nurture’ in the nature–nurture debate was implicitly taken to mean shared environment because from Freud onwards, theories of socialization had assumed that children’s environments are doled out on a family-by-family basis. In contrast, the point of non-shared environment is that environments are doled out on a child-by-child basis. Note that the phrase ‘non-shared environment’ is shorthand for a component of phenotypic variance—it refers to ‘effects’ rather than ‘events’, as discussed later.
The 1987 paper reprinted in this issue of the International Journal of Epidemiology1 brought together evidence for the importance of non-shared environment in the development of personality, psychopathology and cognitive abilities, expanding on a previous paper.2 The purpose of the present commentary is to reflect on non-shared environment three decades after the topic emerged. Progress and problems in studying non-shared environment were reviewed in 2001;3 rather than providing a systematic update of this burgeoning field, my current goal is to suggest some new directions for research in this area.
The 1987 paper was published with 32 commentaries and our response,4 which I recommend. These commentaries and the response to them raised many of the issues that resurfaced during the following decades, such as the following:
Non-shared environmental effects need to be distinguished from error of measurement (yes).
Non-additive genetic variance can account for non-shared environmental effects (no).
Genotype–environment interaction and correlation can account for non-shared environmental effects (no).
Prenatal factors can contribute to non-shared environmental effects (yes).
Non-shared environmental effects may be more influential in extreme situations such as abusive families (yes).
Perceptions of environment can be an important source of non-shared experience (yes).
Non-shared environment can involve chance in the sense of idiosyncratic experiences (yes).
It is noteworthy that none of the 1987 commentaries disagreed with the fundamental phenomenon that children growing up in the same family are very different. There was also general agreement that most of the environmental variance is of the non-shared variety. As I reflect on the following decades of research on non-shared environmental influence, the basic finding of the importance of non-shared environment has not been seriously challenged, which seems surprising to me given its far-reaching implications for understanding how the environment works. It should be emphasized that the message is not that all environmental variance for all traits is non-shared but rather that most environmental influence for most traits is non-shared. Some significant shared environmental variance has been found for some traits.5,6 For example, for antisocial behaviour in adolescence, shared environment accounts for ~15% of the total phenotypic variance, although even here non-shared environment accounts for much more of the variance, ~40%.7 Another example of significant shared environmental influence is academic achievement, where the effect is surprisingly modest in its magnitude given that this result is based on siblings growing up in the same family and being taught in the same school, often by the same teacher in the same classroom.8 Intelligence (IQ) is a third example, first mooted in the 1987 paper, showing significant shared environmental influence in childhood that diminishes to insignificant levels by adolescence to be subsumed by genetic and non-shared environmental influences, a suggestion subsequently confirmed in several studies and meta-analyses.9,10
A search for ‘nonshared environment OR non-shared environment’ using ISI Web of Science (February 2010) yields 371 entries, after excluding a few inappropriate entries. These 371 entries certainly underestimate the total number of papers in the area—e.g. the search does not even identify our 1987 paper or 29 of the 32 connected commentaries. In part this is due to the fact that non-shared environment has been called by other, less searchable names such as unique, specific and individual environment. Nonetheless, a ‘citation report’ of the 371 entries (Figure 1) is interesting for three reasons. First, it was not until the mid-1990s, a decade after the first papers, that non-shared environment began to be cited. Second, since then, citations of non-shared environment have steadily increased to more than a thousand per year by 2008.
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Citations per year for 371 entries for ‘nonshared environment OR non-shared environment' from ISI Web of Science (March 2010)
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