The parity thesis has been the focus of considerable discussion and response. In a helpful paper, Ulrich Stegmann shows that the parity thesis is really a cluster of theses rather than a single thesis (Stegmann 2012). Some ways of interpreting parity make the idea quite uncontroversial, as no more than an insistence on the complex and interactive character of development, or as pointing to the fact that just as genes come in slightly different versions, with slightly different effects (holding other factors constant), the same is true of nongenetic factors. Epigenetic markers on genes, due to nutritional environments, litter position and birth order, may also come in slightly different variants, with slightly different effects. Other versions of the claim are much more controversial.