3 - Lost in correlations? Direct and indirect genetic causes

Summary

Unless you are willing simply to deny that causality is a meaningful concept then you will need some way of studying causal relationships when randomized experiments cannot be performed. Maintain your skepticism if you wish, but grant me the benefit of your doubt. A healthy skepticism while in a car dealership will keep you from buying a “lemon.” An unhealthy skepticism might prevent you from obtaining a reliable means of transport.

Bill Shipley

The fact that organisms with genotype G₁ may have phenotype P₁ more often than organisms with genotype G₂ is not necessarily a good indication that a G-difference is directly responsible for a P-difference. It may well be that (for whatever reason) G₁ organisms simply find themselves more frequently in environment E₁ than G₂ organisms, and that it is E₁ which causally leads to phenotype P₁ (while E₂ produces P₂). This phenomenon is called “genotype–environment correlation.” It has been extensively discussed in the behavior genetics literature, but it has also been widely used in methodological criticisms of heritability.

The expression “genotype–environment correlation” usually refers to the situations where two separate sources of phenotypic variance (genetic and environmental) happen to be correlated. Sometimes, however, it is interpreted more broadly to cover also the cases where there is a correlation between a genetic and environmental characteristic, even when the genetic characteristic is not directly influencing the phenotype.