 Another difficulty faced by selectionist explanations has to do with their reliability. Glymour has argued against the reliability of explanations constructed using type recursions: “population genetics models evolving populations with the wrong variables related by the wrong equations employing the wrong kinds of parameters” (Glymour 2006: 371). Glymour demonstrates that exogenous fitness variables cannot be used to quantify environmental causes of reproduction that change in their influence during the projection period. Fitness cannot be a kind of summary variable that abstracts away from the causal details of the natural population while also functioning to reliably predict and explain the dynamics of the systems to which type recursions apply. Gildenhuys (Gildenhuys 2011) provides a response to Glymour, acceding that his arguments are valid but contending that there are a variety of ways to complexify type recursions to formally handle the influence of environmental causes; Glymour (2013) provides a reply.