A third source of developmental differences

Abstract

An illustrative list is presented of human and animal studies which each point to the existence of a third source, in addition to genetic and environmental factors, underlying phenotypic differences in development. It is argued that this third source may consist of nonlinear epigenetic processes that can create variability at all phenotypical-somatic and behavioral-levels. In a quantitative genetic analysis with human subjects, these processes are confounded with within-family environmental influences. A preliminary model to quantify these influences is introduced.

Genetic and environmental influences on human psychological differences

“…Composites thus formed likely yield more valid, more reliable, and more heritable measures than single-time self-reports. Another possible source of nonshared environmental variance in ordinary personality traits is chance or developmental noise (Molenaar et al, 1993;Finch and Kirkwood, 2000). It may well be that so many minor chance and idiosyncratic factors intervene between conception and the full manifestation of psychological phenotypes that a meaningful portion of the variance will never be accounted for by specific effects that generalize across individuals.…”

Why are Children in the Same Family So Different? Nonshared Environment a Decade Later

“…We also need to con sider the gloomy pros pect that chance con trib utes to non shared en vi ron ment in terms of ran dom noise, idio syn cratic ex pe ri ences, or the sub tle in ter play of a con cate na tion of events (5,10,11,(56)(57)(58). Fran cis Gal ton, the founder of be hav ioural ge net ics, sug gested that non shared en vi ron ment is largely due to chance, when he com mented that "the whim si cal ef fects of chance in pro duc ing sta ble results are com mon enough."…”

Person-Specific Non-shared Environmental Influences in Intra-individual Variability: A Preliminary Case of Daily School Feelings in Monozygotic Twins

“…There are a few possible explanations to this inter-individual heterogeneity in person-specific non-shared environmental influences. First, stochastic intra-individual developmental noise can lead to inter-individual differences in genetic and environmental influences as a third source, which however cannot be distinguished from non-shared environmental influences in conventional behavioural genetic studies (e.g., Molenaar et al 1993;Molenaar and Raijmakers 1999). Second, epigenetic processes (e.g., DNA methylation) involving interactions between genes, environment, and other processes could also affect gene expression to lead to interindividual heterogeneity (e.g., Bell and Saffery 2012;Bell and Spector 2011;Dolan et al 2015;Fraga et al 2005;Kan et al 2010;Petronis 2010;Wright et al 2014).…”

“…For example, behavioural genetic studies adopting mixture modelling have demonstrated population heterogeneity regarding genetic and environmental influences (e.g., Eaves et al 1993;Gillespie and Neale 2006;Muthén et al 2006;Neale 2014). Stochastic intra-individual developmental noise (e.g., Molenaar et al 1993;Molenaar and Raijmakers 1999), gene expression and epigenetic processes (e.g., DNA methylation) can also result in substantial inter-individual differences in genetic and environmental influences (e.g., Bell and Saffery 2012;Bell and Spector 2011;Dolan et al 2015;Fraga et al 2005;Kan et al 2010;Petronis 2010;Wright et al 2014). Therefore, investigation of intra-individual variability could potentially provide unique and important information regarding genetic and environmental influences on intra-individual developmental processes that conventional behavioural genetic studies focusing on inter-individual differences could not offer, especially at the individual level.…”