 Some biologists and philosophers of biology have been motivated by problems with the backward-looking etiological approach, or by seeing examples from biology that seek to identify the present functions of a trait. To deal with these issues they propose a dispositional or forward-looking approach that analyzes function in terms of those effects it is disposed to produce that tend to contribute to the present or future maintenance of the trait in a population of organisms. Various ways of spelling this out include Hinde’s (1975) account of strong function, Boorse’s (1976, 2002) biostatistical theory, Bigelow and Pargetter’s (1987) propensity theory, and Walsh’s (1996) relational theory (see also and Walsh &amp; Ariew 1996). 