In his 1994 book “Why Zebras Don’t Get Ulcers,” Robert Sapolsky described the difference between the types of stress that people often experience and the ones that other animal species do. In his titular example, if you were a zebra, you’d most likely face acute physical stress – the lion about to eat you – which requires immediate physiological adaptations (the fight-or-flight response). A second type of stress might be chronic and physical (drought, famine, parasites, etc.).

However, the third type of stress on Sapolsky’s list – the type most prominent in an industrialized human’s life – was social and psychological. While our species certainly benefited from expanding brain size over the last few million years, it too came with trade-offs, including the ability to overthink and worry about things to come down the road. This type of stress would not have featured too prominently into the mental lives of other species. As Sapolsky wrote:

“How many hippos worry about whether Social Security is going to last as long as they will, or what they are going to say on a first date? …

For the vast majority of beasts on this planet, stress is about a short-term crisis, after which it’s either over with or you’re over with. When we sit around and worry about stressful things, we turn on the same physiological responses – but they are potentially a disaster when provoked chronically. A large body of evidence suggests that stress-related disease emerges, predominantly, out of the fact that we so often activate a physiological system that has evolved for responding to acute physical emergencies, but we turn it on for months on end, worrying about mortgages, relationships and promotions” (p. 5 – 6).

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