Conditional relations by monkeys: Reflexivity, symmetry, and transitivity.

Two lab monkeys learned a simple match game. A sample picture appeared on a screen. Two choice pictures followed. If the monkey touched the picture that went with the sample, a food pellet dropped.

The team never taught every possible pair. They only trained a few. Then they tested if the animals could pick the correct picture for brand-new pairs. This checks for stimulus equivalence: reflexivity (A=A), symmetry (if A=B then B=A), and transitivity (if A=B and B=C then A=C).

After only a handful of training trials, both monkeys correctly matched never-before-seen pairs. They acted as if they understood the hidden rules. The results showed full stimulus equivalence in a non-human subject for the first time.

Marr (1989) threw cold water on the party. That paper says the monkeys were probably just repeating old response patterns, not showing true equivalence. The critique is useful: it tells us to add extra test controls so we don’t mistake habit for learning.

Later work extended the idea to other species. Busch et al. (2010) got pigeons to show reflexivity after special training. Swisher et al. (2018) went further: pigeons showed reflexivity even without identity-matching drills. Each study tightens the method and keeps the core finding alive.

Noell et al. (2026) moved the logic to children. Preschoolers learned hierarchical categories and then named new items without direct teaching. The monkey data opened the door; now we use the same rules to build language and academic skills with kids.

You now have a road map for creating new relations without drilling every single one. Start with a few conditional-discrimination trials, probe for emergent relations, and add controls to rule out rote patterns. This saves teaching time and builds flexible learner repertoires, whether your client is a monkey, a preschooler, or a student with autism.