No, humans are not the only rational beings.

Humans are rational beings. We have the capacity to identify relevant information for decision-making; to evaluate different pieces of information and choose the one we consider most reliable; and, when faced with contradictory clues or evidence, we can weigh them to ultimately choose the best option.

Until now, we had thought we were the only animals capable of doing this. And yet, new research led by researchers from the University of St Andrews in Scotland, and published in the magazine Science, It demonstrates that other primates, chimpanzees, our closest evolutionary relatives, also make decisions based on the reliability of information and can change their minds in the face of new clues.

"They are rational beings," he asserts. Catalan primatologist Josep Call, co-author of the study. In philosophy, this expert reminds us, rational beliefs are considered to be those based on evidence, and when new evidence emerges, one can change their mind. And this, this researcher points out, is precisely what they have observed chimpanzees doing.

Experiment in a chimpanzee sanctuary

Until now, thanks to studies like those conducted by Call, it was known that non-human apes could use indirect evidence to make decisions, such as which box contained food based, for example, on the noise made when it was shaken. What was unknown was whether they were capable of making decisions by evaluating clues of varying reliability and even revising their choices when confronted with contradictory evidence or information. This is what this study resolves. The researchers conducted a series of experiments with a group of 15 to 23 young chimpanzees living in a sanctuary for these primates located on Ngamba Island, Uganda. The apes, who participated entirely voluntarily, were presented with wooden boxes with glass lids, some of which contained a reward, such as fruit or handfuls of nuts, and they had to choose one from among the different options. To do so, they had to assess the reliability and weight of each piece of evidence. For example, in one of the experiments, they were shown two identical boxes. In one experiment, researchers placed a piece of apple in the box and gave the chimpanzees a strong cue—turning the box containing the fruit so they could see it—and then shaking the other box to show it also contained something. The chimpanzees consistently chose the box where they had seen the apple. Visual evidence prevailed over auditory evidence.

In another case, the researchers again presented them with two boxes: they shook one containing peanuts and placed a small handful of peanuts in front of the other. In this situation, most of the chimpanzees chose the box that made the noise. They considered the auditory evidence to be the strongest. But the experiments also included more complex situations. They were given a choice between three boxes: in one, they were shown how to arrange fruit; in another, they shook it and made a noise as if it contained food; and in the third, they were given no information. Once the primates had been shown all three boxes, the scientists removed the first one, which contained the strongest evidence. In that case, the animals chose the box that made the noise.

Capable of rectifying

The researchers also found that the chimpanzees were able to revise their choices when they realized that information was incorrect. In one of the boxes, they saw what appeared to be an apple, but the researchers then showed a photograph of an apple. The primates were then able to take this evidence and conclude that what they had seen might not have been a real apple, and consequently chose the second box. "It's a much more complex reasoning," Call points out, because "it involves comparing different sources and types of evidence and also judging their quality." "What looked like an apple wasn't actually an apple, and they are able to integrate this information into their decision." Now, in a new series of experiments, the researchers are trying to determine whether this capacity for rational reasoning is innate in humans or develops after a certain age. They are also investigating what happens when the chimpanzees have to choose between their own beliefs and those of another individual. Are they able to change their minds?

We humans always like to think that we are "the rational animals par excellence, and it's true," says Call, but what these experiments demonstrate is that "some elements of what we consider human rationality already exist in other species, at least in chimpanzees." And this primatologist affirms that it is very likely that they will appear in other species, such as dogs.