“…Equivalence relations (Sidman, 1986), which take the form of reflexivity, transitivity, and symmetry, also form a stimulus class. Aside from evidence of transitivity in nonhuman animals (Boysen, Berntson, Shreyer, & Quigley, 1993;D'Amato, Salmon, Loukas, & Tomie, 1985;Yamamoto & Asano, 1995) and, in some cases, claims of symmetry (Cerutti & Rumbaugh, 1993;Kojima, 1984;Savage-Rumbaugh, 1986;Tomonaga et al, 1991; but see also Sidman et al, 1982;Yamamoto & Asano, 1995), equivalence relations are not well established in nonhuman animals (McIntire, Cleary, & Thompson, 1987;Schusterman & Kastak, 1993;Sidman, 2000;Zentall & Urcuioli, 1993; see also Hayes, 1989). For 2 chimpanzees in the current experiment (Sherman and Lana), symmetry was not evident when they were younger and had been exposed to standard symmetry tests (Dugdale & Lowe, 2000).…”