When you hear hoofbeats, think horses, not zebras
Originated in: 🇺🇸 United States of America
Don't look for zebras on Greene Street
Earliest attestation: Dr. Theodore Woodward, University of Maryland School of Medicine (late 1940s)
This is a saying reminding people to look for the most common explanation for something first, rather than jumping to some exotic cause. If you hear a hoofed animal running behind you, it could theoretically be a horse, a zebra, or even a zorse. They are all the same kind of animal, and for most people there is no way to tell the difference between them sound alone despite their striking visual differences. However, unless you live in southern or eastern Africa, zebras are incredibly rare. Unless you are at a zoo, there is essentially zero chance that you are hearing a zebra. Thus, when you hear hoofbeats, you should assume it is a horse and not a zebra.
This saying originated at the University of Maryland School of Medicine when the late Dr. Theodore Woodward reminded his students to not look for zebras on the street outside of the school if they hear an equine galloping by. The point he was trying to make was that if a patient is presenting symptoms that could be attributable to either a common ailment or a rare disease you once read about in a medical journal, a doctor should assume it is the common condition first. Eliminate the most likely possibilities first, and then move on to less likely ones. Statistically, this leads to the most correct diagnoses.
It is still commonly used in the medical field and is applicable to medical diagnosis in general. If you have a headache and read online that that’s a symptom of brain cancer, you should not assume such an unlikely, worst-case scenario. You should first rule out the far more common causes: Are you drinking enough water? Are you going through caffeine withdrawal? Are you under too much stress?
It can also be applied to any area of life however. If you hear a strange noise in your house, you should first assume it is a common cause like a pipe rattling or the foundation settling before you assume it is an axe-murderer. If a calculator gives you the wrong answer, assume you hit the wrong number at some point before assuming the calculator is defective. If your spouse is oddly quiet, assume he or she is having a bad day before assuming he or she is cheating on you. The most likely, boring cause should be assumed before something abnormal. This not only leads to being right more often, but also just to have a less stress-filled life as the most common explanations are usually less worrying than their extreme counterparts.
It is worth noting that this is just a heuristic. Sometimes rare things do happen, but it is not an efficient use of time to focus on things first. That said, once you have ruled out the most common causes, you then do need to examine the less likely ones no matter how unlikely it may have been to have to. One cannot focus solely on the most frequent causes, just one should focus on them primarily.