Researchers at Stanford combined Union Army veteran records from the Civil War era with two modern cohorts, assembling 677,423 temperature readings that stretched across 157 years of birth years.
Men born in the early 1800s ran about 0.59 degrees Celsius hotter than men measured in recent decades. Temperature fell steadily by roughly 0.03 degrees per birth decade in both sexes, even after the team adjusted for age, body size and time of day.
The team's leading explanation is a quieter body, not a different thermometer: less chronic infection and inflammation, better dental care, and climate-controlled homes that no longer force the body to burn extra fuel to stay warm. A separate study of an Amazonian population without modern medicine found a similar decline, which complicates a purely medical explanation.
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98.6 was never a fixed number.
Source
- Protsiv, M., Ley, C., Lankester, J., Hastie, T., & Parsonnet, J. (2020). Decreasing human body temperature in the United States since the Industrial Revolution. eLife, 9, e49555. https://doi.org/10.7554/eLife.49555