 The multiple inheritance system perspective highlights a variety of questions (see  section 4)  and many fundamental questions remain open. Some of them depend on empirical work, perhaps most importantly determining the prevalence and stability of transgenerational epigenetic inheritance (Helanterä &amp; Uller 2010; Jablonka &amp; Raz 2009; Shea et al. 2011). The developmental aspects of many of the inheritance systems discussed in this entry, in particular behavioral inheritance, are still not fully understood. Generally, many open questions remain about the interactions between the various systems and about their evolution, in particular the evolution of social learning and the evolution of language. A crucial element downplayed by most contemporary accounts is the connection between population level issues, such as population size, mating strategies, etc., and the properties of inheritance systems. Addressing these issues requires quantitative modeling, and eventually the integration of multiple inheritance systems with their different characteristics into population genetics (see Day &amp; Bonduriansky 2011, Geoghegan &amp; Spencer 2012).