Comprehension of "absence" by an African Grey parrot: Learning with respect to questions of same/different.

One African Grey parrot named Alex learned to say 'same,' 'different,' or 'none' when shown two objects. The trainer first taught Alex to label colors and shapes. Then she asked 'What's same?' or 'What's different?' after showing new pairs. If nothing was the same or different, Alex had to say 'none.'

Alex got about three out of four answers right on brand-new pairs. He even said 'none' when the items had no difference. This shows a bird can learn an abstract rule: report when something is missing.

Sigurðardóttir et al. (2012) later showed college adults can do the same thing with made-up Icelandic words. They formed equivalence classes almost without errors, proving the rule works in humans too.

Richardson et al. (2025) pushed the idea even further. Rats learned equivalence with smells instead of pictures. They used simple reversal training and still built classes, showing the skill is not just for talking animals.

Murphy et al. (2005) brought the idea to kids with autism. After equivalence training, the children asked for new items they had never directly been taught to request. The parrot study opened the door for all these later uses.

You now know that conditional-discrimination training can create brand-new, untrained responses in almost any learner: birds, rats, college students, and kids with autism. When you teach same/different or yes/no rules, build in 'none' or 'absent' trials early. The learner will tell you when a feature is missing, and you can skip extra drills.