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# Primate Statistics

## Are humans the only ones crunching numbers?

Source: Franziska Marke Nieder, used with permission

We are obsessed with numbers. At this particular moment, they seem to be everywhere—the number of people infected by COVID-19, the number of tested people, and the number of the dead. Remembering what we learned in school about probability and statistics, we can try to calculate the death rate on our own—but it turns out the numbers do not tell us much. They are not comparable between countries, and one important number is missing: the real number of people who have (or had) the virus.

Calculating percentages only makes sense if you really know what 100 percent is. Even chimpanzees know that.

Why? Probably not because chimpanzees study the death rates of their closest living relatives and their greatest threat. But it is likely that their sense for intuitive statistical reasoning developed for the same reasons it did in humans. This ability enables chimpanzees (and us) to infer general regularities from finite numbers of observations. In other words, it helps them (and us) to predict the outcome of single events using prior information.

And how do we know? Researchers from St. Andrews (UK) and Göttingen (Germany) developed a paradigm in which chimpanzees were presented with yummy food (peanuts) and acceptable food (carrot pieces). Chimps were presented with two transparent buckets filled with mixed ratios of peanuts and carrot pieces of roughly equal size and shape. One bucket contained a ratio that was more favorable than the other in terms of its proportion of peanuts to carrots.

The experimenter drew one item out of each of the buckets in a way that the chimpanzee could not see what was drawn. While the two items were hidden in the fists of the experimenter, the chimpanzees had to make a choice. Thus, the task required chimpanzees to reason where the peanut was—which, of course, depended on the ratio of peanuts and carrots in the buckets.

With an overview of the contents of the buckets, the chimpanzees had the important information—what 100 percent of the food items were (the sum of peanuts and carrots)—and so they could make a good guess. And they are not the only ones. Bonobos, gorillas, and orangutans performed similarly in such a test—and so did humans from a German university (Eckert et al. 2018, Rakoczy et al. 2014).

Similar to other primates, we are intuitive statisticians, and thus, we can predict outcomes. But we can only do that when we use the correct numbers. Even then, we are talking about probability—not certainty. The tested apes could never be sure that they would receive the peanut, and neither can we be sure if we will get the virus or not. Thus, maybe it is better to be less obsessed with the correct numbers (that are still missing) and to start looking at which “buckets” in our lives have the most peanuts and enjoy these calm days of spring.

References

Eckert, Johanna, Josep Call, Jonas Hermes, Esther Herrmann, and Hannes Rakoczy. 2018. "Intuitive statistical inferences in chimpanzees and humans follow Weber’s law." Cognition 180:99-107. doi: https://doi.org/10.1016/j.cognition.2018.07.004.

Rakoczy, Hannes, Annette Clüver, Liane Saucke, Nicole Stoffregen, Alice Gräbener, Judith Migura, and Josep Call. 2014. "Apes are intuitive statisticians." Cognition 131 (1):60-68. doi: https://doi.org/10.1016/j.cognition.2013.12.011.