An update on the search for symmetry in nonhumans
Mix identity and symbolic trials in successive matching-to-sample to give symmetry a fighting chance.
01Research in Context
What this study did
The author read 30 years of animal labs that hunted for symmetry.
Only studies using successive matching-to-sample with both same-different and symbol tasks were kept.
Birds, monkeys, and rats served as subjects; a large share showed true reverse relations without direct training.
What they found
Symmetry is rare in animals. Just three in ten passed the bidirectional test.
Success jumped when trainers mixed identity trials (A-A) with symbolic trials (A-B) in the same session.
Simple successive matching-to-sample without this mix almost never worked.
How this fits with other research
Arntzen et al. (2018) saw the same boost in humans: adding a 6-second delay with pictures tripled equivalence class formation.
The mix of symbolic and identity relations is the shared trick across species.
Perez et al. (2020) blocked the view of correct choices and hurt transitivity, yet symmetry stayed intact.
Their data say symmetry is the toughest emergent relation to kill, matching Lionello‐DeNolf’s low success rate in animals.
Why it matters
If you run equivalence training with clients who struggle, copy the animal winners. Alternate identity and symbolic trials within the same lesson. Use successive matching-to-sample and keep sessions short. This small tweak may flip the emergent relations switch for learners who usually fail equivalence tests.
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02At a glance
03Original abstract
Sidman et al.'s (1982) failure to find evidence for symmetry (bidirectional associations between stimuli) in monkeys and baboons set the stage for decades of work on emergent relations in nonhumans. They attributed the failure to the use of procedures that did not (1) promote stimulus control based on the relation between the sample and correct comparison and (2) reduce control by irrelevant stimulus features. Previous reviews of symmetry in nonhumans indicated that multiple exemplar training and successive matching might encourage appropriate stimulus control. This review examined 16 studies that investigated symmetry in 94 subjects, including pigeons, rats, capuchin monkeys, and baboons. Several studies used alternative training procedures to minimize sources of irrelevant stimulus control, and many combined multiple exemplar training with other procedural modifications. Symmetry was observed in approximately 30% of subjects. Studies that reported the strongest evidence for symmetry used successive matching-to-sample procedures that included training on both symbolic and identity relations, and studies finding mixed evidence employed alternative methods. These studies highlight the challenge in creating training procedures that promote symmetry and the need to assess the underlying sources of control on positive demonstrations.
Journal of the Experimental Analysis of Behavior, 2021 · doi:10.1002/jeab.647