In this little game, humans—regardless of age, education or culture—almost always win (although the error rate is still 40% for the "least regular" series).
Baboons, by contrast, still have an error rate of at least 50%, even after training; they do not seem to distinguish regular figures from irregular ones. Humans may therefore possess a capacity for symbolic abstraction unknown to other primates.

Find the odd one out in each of these three series!

Reference: Geometry, a human intuition. Sylvie Rivière, Les défis du CEA 245, September–October 2021, available online.