The variance attributable to the shared environmental effects by family members was estimated for HTK and ASF with ASSOC.
The unique environmental structure was first examined, followed by the development of shared environmental influences and finally by the structure of the genetic influences.
However after PAEE was adjusted for weight in the study by Franks et al. the proportion of variance explained by shared environmental factors was remarkably similar to the variance observed in this study (69% vs 73%) [19].
To test this hypothesis we used bivariate variance component analysis, which evaluates to what extent the observed phenotypic correlation can be caused by pleiotropic genetic factors (measured by additive genetic correlation, rG), and/or by shared environmental factors (measured by environmental correlation, rE).
Our finding suggests that a single infectious pathogen is being spread in study households; however, whether the infections are caused by a shared environmental source or secondary transmission from an infected household member is unclear.
Harden and Mendle (2011) used Add Health data within a biometrical design and found that the relationship between intelligence and AFI was fully accounted for by shared environmental differences, suggesting at least the location of the causal mechanism — the part of the household environment shared by siblings that influences both child intelligence and AFI.
In our study, it is noteworthy that more covariance between autistic traits and language could actually be explained by shared environmental influences, as indicated by the bivariate heritability and environment estimates.
Based on the combined data from both cohorts 55% (95%CI: 36% - 75%) of the total variance in liability to smoking is explained by genes, 23% (95%CI: 7% - 39%) by shared environmental factors and the remaining variance of 21% (95%CI: 16% - 28%) is explained by unique environmental factors.
None of the five leisure time activities were influenced by common shared environmental contributions.
In summary, this study concerning the aetiology of homophobic attitudes revealed that familial aggregation in attitudes toward homosexuality is accounted for by genes as well as by shared environmental factors.
