“Tiger got to hunt, bird got to fly; Man got to sit and wonder, ‘Why, why, why?’ Tiger got to sleep, bird got to land; Man got to tell himself he understand.”
-Kurt Vonnegut

A study in the journal Psychological Science proposed that monkeys have the ability to engage in abstract reasoning through disjunctive syllogisms – an ability that was previously thought to be unique to humans and require language.

This study was conducted by Dr. Stephen Ferrigno at Harvard University, Dr. Yiyun Huang at Yale University, and Dr. Jessica Cantlon at Carnegie Mellon University.

Logical reasoning has played an important role in human evolution. By some accounts, our ability to think in abstract terms paved the way for all the complex systems that are unique to the human experience.

One example of logical reasoning humans engage in is the disjunctive syllogism – if we know that either A or B is the case, if it is not A, then it must be B. If someone flips a normal coin and tells you that it did not land on tails, you would logically reason that it must have landed on heads.

Previous studies have used interesting methods to determine just how young humans develop this ability. By hiding a desired item under one of two cups, showing the young participants that one cup is empty, and then seeing if they check under the remaining cup, researchers have found that children as young as 2 engage in this type of behavior. Interestingly, researchers using similar paradigms have found that apes also engage in this behavior.

However, one of the problems that Drs. Ferrigno, Huang, and Cantlon note about this two-cup study design is that it is possible that participants treat each cup as if it might have a reward under it, independent of the contents of the previous cup. Just by showing an interest in the second cup does not necessarily mean that a participant understands that the reward had to be under the second cup. As such, this task is not a good assessment of a person’s (or animal’s) ability to logically reason their way through disjunctive syllogisms.

Using a slightly more complicated design to better assess participants’ reasoning abilities (with two desired objects and two independent sets of two cups), previous research has found that the ability to reason through disjunctive syllogism does not emerge in humans until around the ages of 3 to 5. These findings are more in-line with the theory that working through a disjunctive syllogism is uniquely a human ability that requires an understanding of language.

The purpose of the current study was to use the more valid four-cup paradigm to test whether monkeys can reason through disjunctive syllogisms. The researchers explain that if any of the monkeys in the current study demonstrate this ability, it would be an existence proof that this type of reasoning is not a distinctly human cognitive feature.

The researchers started the study by training nine adult baboons on how to participate in the experiment (and receive delicious grapes as rewards!). Of these nine baboons, four demonstrated the required level of understanding to continue with the experiment.

The task involved two sets of two cups and two grapes – one for each set of cups. The experimenter would first show the baboon that one of the grapes was going under one of the first two cups but hid which one by using a blocking contraption. Then, the blocking contraption was moved over to the second set of two cups and the baboon was shown a second grape placed under one of those two cups.

If baboons can reason through disjunctive syllogisms, then picking an empty cup first should lead them to stick to that same set of cups, knowing that the second cup of that set must contain the grape. If they pick a grape-containing cup first, that should lead them to switch sets, knowing that the second cup of that first set must be empty.

The results suggested that baboons can in fact reason through disjunctive syllogisms: they would stick to the same side if they picked an empty cup first, and they would switch sides if they picked a grape-containing cup first. These tendencies occurred significantly more frequently than would be predicted by chance.

The researchers also observed that baboons even engaged in unexpected ‘pre-pointing’ behaviors. They often would immediately point at their second choice after seeing the first cup was empty, but before they were given the indication that it was time to make a second choice, which suggests they have the ability to immediately update their knowledge of where the grape must be given that the first cup was empty.

These findings contradict the theory that this type of syllogistic reasoning requires a linguistic understanding of the operators ‘or’ and ‘and’. It also demonstrates that some non-human primates have the cognitive ability to compare abstract representations of different possible outcomes to inform their decisions.