Evolutionary processes, such as genetic drift and natural selection, can cause allele frequencies to differ between populations.
Twin and adoption studies have provided much of the evidence for gene–environment correlations by demonstrating that putative environmental measures are heritable.
Studies in which researchers have measured child-specific aspects of the environment have also shown that putative environmental factors, such as parental discipline or warmth, are moderately heritable.
Like parental discipline, these health related behaviors are genetically influenced, but are thought to have environmentally mediated effects on disease.
To the extent that researchers have attempted to determine why genes and environments are correlated, most evidence has pointed to the intervening effects of personality and behavioral characteristics.
A recent study found that children were almost 2.5 times more likely to be diagnosed with attention-deficit hyperactivity disorder (ADHD) if their mothers were divorced, separated, or never married.
In this sample, however, mothers possessing the short allele of the dopamine receptor gene DRD2 were more likely to be divorced, separated, or never married.
Therefore, part of the association between parental marital status and ADHD diagnosis among children in this sample is due to the confounding variable of maternal DRD2 genotype.