Testing for symmetry in the conditional discriminations of language-trained chimpanzees.

Dugdale et al. (2000) worked with two language-trained chimpanzees. The team first taught each chimp many A-B matching tasks. Then they gave repeated symmetry tests: show B, pick A. Every correct pick earned food.

The goal was to see if lots of reinforced symmetry examples would make the reverse relation pop out. Earlier work said yes; this study checked that claim.

Both chimps failed. They could do A-B, but when B popped up they rarely picked A. Extra symmetry drills and pay-offs did not fix the gap.

The result knocks down the idea that sheer exemplar training is enough to create symmetry in non-humans.

Hopkinson et al. (2003) looked back at 55 people with profound ID and little or no speech. Most passed equivalence tests. The review even lists earlier chimp data, so the 2000 failure sits inside a bigger picture: humans without language can still show symmetry, chimps mostly don’t.

Diaz et al. (2020) gave college kids brief vocal training after standard conditional-discrimination drills. Fifteen of sixteen then passed derived relations. The short verbal step made the difference—something chimps can’t do.

Shawler et al. (2022) showed that once equivalence forms in adults, reinforcing power transfers too. Again, the classes held. Taken together, the human studies say: language or other mediating cues tip the scale toward symmetry; exemplars alone are not enough.

If you work on equivalence with clients who have limited verbal skills, don’t assume dozens of reinforced reversals will guarantee emergence. Add mediators—names, signs, or other cues—that the learner can use. Probe symmetry early and often; stop drilling if it doesn’t emerge and adjust the program instead of hoping more trials will do the trick.