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Everybody knows that the average human body temperature is 37°C – but everybody is wrong. It turns out that the bodies of people in the US have been cooling since the 1860s.

Physicians who have studied body temperature , says Julie Parsonnet at Stanford University in California. “But they’ve always thought that it was just measurement error in the past, not because temperature had actually dropped.”

To find out what really happened, Parsonnet and her team combined three data sets. The first covered 23,710 Union Army veterans from the American Civil War, whose temperatures were measured between 1860 and 1940. “It took me a long time to find a database back to the 19^th century that had temperature in it,” says Parsonnet. The other data sets spanned 1971 to 1975 and 2007 to 2017. In total, the team analysed 677,423 temperature measurements.

On average, American body temperature has declined by 0.03°C per decade. Men born in the early 19^th century had body temperatures 0.59°C higher than men today. The data for women doesn’t go as far back, but their body temperature has dropped 0.32°C since the 1890s. That means average body temperature today is about 36.6°C, not 37°C as widely thought.

Parsonnet offers two pieces of evidence that the fall is real and not simply the result of older thermometers being unreliable. First, the cooling trend is visible within the more modern data sets, in which the thermometers used were presumably more reliable. “The decline we saw from the 1860s to 1960s, we see the same decline from the 1960s to today,” says Parsonnet. “I don’t think there’s much difference in the thermometers between the 1960s and today.”

Second, older people were found to have higher body temperatures than younger people measured in the same year, regardless of when that year was. You would expect to see further differences in data if the thermometers were less accurate.

That wasn’t the case, and grouping measurements by subjects’ dates of birth did line up with the changing temperatures. That means changing thermometers alone can’t account for the effect.

“The most likely explanation in my view is that, microbiologically, we’re very different people than we were,” says Parsonnet. Modern people have fewer infections, thanks to vaccines and antibiotics, so our immune systems are less active and our body tissues less inflamed. If that is true, body temperatures should also have fallen in other countries where people’s health has improved.

The cooling trend shows no sign of stopping soon, says Parsonnet. “There is going to be a limit, we’re not going to get down to zero,” she says. “But I just don’t know where that is.”