Heritability and its Discontents

In an era of genome-wide association studies (GWAS), where in excess of half a million genetic markers are used to isolate genetic loci associated with a multiplicity of traits, what hope is there for the venerable heritability (h2) statistic? Genetically informative twin and adoption samples can continue to be used to explore the effects of cultural transmission, assortative mating and gene-environment interplay on developmental processes. However, there is growing evidence that the genetic architecture of complex behavioural traits consists of the combined effect of many hundreds, if not thousands, of small effect loci-a model sometimes referred to as quasi-infinitesimal. If this is correct, then we will need much larger sample sizes than currently considered-perhaps consisting of hundreds of thousands of individuals. Although this may seem daunting at present, advances in genotyping technology suggest that we will soon see case-control samples of tens of thousands subject to genome sequencing.