This 2015 international study of intellectual disability used human twins to estimate the impact of genetic, shared-environment, and non-shared-environment on the study’s subjects:
- “Estimate of 0.46 (95% CI: 0.32–0.60) can be ascribed to genetic factors.
- Estimate of 0.30 (95% CI: 0.19–0.41) may be due to environmental factors involved in growing up in the same environment.
- The remaining 24% (95% CI: 0.18–0.29) of the difference is due to error of measurement and nonshared environmental influences.”
The primary causes of individual differences in DNA methylation are environmental factors used analysis of the study’s twin subjects’ CpG methylation compared to “CpGs displaying differential methylation in a healthy population (pDMCs)” to estimate:
- “37 % of the pDMCs genetic effects
- 3 % of the pDMCs had shared environment
- The remaining proportion of the non-genetic variance was due to non-shared environment and/or stochastic factors.”
Those researchers performed several additional tests to find and confirm:
“Non-shared environmental DMCs account for 64% of all detected DMCs.”
Comparing the two studies, the current study’s 32%-60% estimate of genetic effects encompassed the second study’s 37% estimate. However, the current study’s researchers treated their 18%-29% non-shared environment estimate as a remainder not warranting further investigation, whereas the second study’s researchers validated their 64% non-shared environment estimate.
Bringing in a third study, a relevant citation from Epigenetic consequences of early-life trauma: What are we waiting for? confirmed the second study’s estimates with a 2000 twin study that found:
“Environmental effects specific to the individual (63%), whilst genetic effects accounted for 37%. Subsequent studies have produced similar results.”
“The human brain is extensively shaped by its environment no matter its genetics.”
The epigenetic effects of each of our unique experiences of our non-shared environment predominately determine our individual physiology.
http://www.pnas.org/content/early/2015/12/23/1508093112.full.pdf “Discontinuity in the genetic and environmental causes of the intellectual disability spectrum”