Symmetry occurs when an individual demonstrates that two stimuli are reversible in a matching-to-sample task without direct training. If a person learns that stimulus A is equivalent to stimulus B (A=B), they will also recognize that stimulus B is equivalent to stimulus A (B=A) without needing additional instruction.
Example
A child is taught to match the word “cat” (stimulus A) to a picture of a cat (stimulus B). Later, without being directly taught, the child can also match the picture of the cat (stimulus B) to the word “cat” (stimulus A). This shows symmetry in their learning.