One intriguing finding from this study that deserves further discussion is the substantial influence of shared environmental effects on the internalizing spectrum. Common environmental influences, particularly in adult samples, have been notoriously difficult to document (Turkheimer, 2000). Standard biometrical modeling on most psychological phenotypes of interest finds that eliminating shared environmental effects from the model does not appreciably decrease model fit. Yet, we found moderate common environmental influences even in the standard, no-moderation model; the biometrical moderation models found an even greater impact of shared environment at the extreme high end of quality. For those with the most positive marital relationships, the majority of the variance in the internalizing spectrum is due to environmental factors shared in common with family members. A possible interpretation of this finding is that a person who develops an internalizing syndrome, even in the context of a fulfilling and satisfying marriage, may have developed poor problem-solving and stress-reducing habits in the context of the childhood home they shared with their siblings – habits that continue into adulthood.
