The Washington PostDemocracy Dies in Darkness

Heat makes you dumb, in four charts

Sweaty man trying to refresh from heat with a fan (tommaso79/iStock)

Man, it’s a hot one.

And that could mean bad news for your performance at work or school, according to at least four recently published studies.

The reports, which examine the effects of air temperature on cognitive performance in the United States and China, rely on different data sets and methods to arrive at the same conclusion: The hotter it gets, the more our brains seem to slow down.

The good news? These effects can be mitigated by air conditioning. But access to air conditioning, particularly in American public schools, is dependent in large part on economic factors, meaning rich kids tend to have the luxury of taking standardized tests in air-conditioned schools while many poor kids do not. And thanks to climate change, rising temperatures in coming years are likely to place even more stresses on kids' — and adults' — cognitive abilities.

In four charts, here’s what the studies show.

Study No. 1: Temperature and High-Stakes Cognitive Performance: Evidence from the National College Entrance Examination in China (July 2018)

The first batch of results comes from a working paper published this month by the National Bureau of Economic Research. A team of American and Chinese researchers looked at the effect of average daily temperature (that is, high temperature plus low temperature, divided by two) on Chinese students' scores on the National College Entrance Examination, a high-stakes standardized test that is “almost the sole determinant for college admission in China,” per the researchers.

Pairing millions of test results with local meteorological data for the days students took the exam, researchers found that, overall, every temperature increase of 3.29 degrees Celsius (or about 5.9 degrees Fahrenheit) reduced students' scores on the exam by 1.12 percent, dampening their odds of getting into the most selective colleges by 1.97 percent.

Chinese authorities are, in fact, well aware that ambient temperature can affect students' test scores: Citing Chinese media reports, the study authors note that the use of air conditioning on testing days in some regions is prohibited “in order to ensure fair competition with regions in which AC is not available.”

As it turns out, that’s a common thread running through all these studies: Air conditioning can eliminate the effects of heat on testing performance entirely. But access to classroom AC isn’t evenly distributed throughout American society.

Study No. 2: Heat and Learning (May 2018)

Earlier this year, a different team of researchers ran a similar inquiry on the effect of heat exposure on American students' PSAT scores. Rather than the average temperature on test day, these researchers were interested in how the cumulative number of hot days before the test might affect students scores.

“Hotter school days in the year prior to the test reduce learning, with extreme heat being particularly damaging and larger effects for low income and minority students,” they found. “Without air conditioning, each 1°F increase in school year temperature reduces the amount learned that year by one percent.”

That’s a staggering finding: All other things being equal, in the absence of air conditioning, a 10-degree increase in school-year temperature can reduce the amount of material learned by students by about 10 percent.

The correlation is so strong it even shows up plainly on a map: In the figures below from the paper, the county-level distribution of 90-degree days bears a striking resemblance to the distribution of PSAT scores.

Generalizing outward from these data, the authors think that if heat can lower test scores, it can reduce overall worker productivity, as well. “Heat exposure can reduce the rate of learning and skill formation, thus potentially reducing the rate of economic growth,” they conclude.

Study No. 3: Reduced cognitive function during a heat wave among residents of non-air-conditioned buildings: An observational study of young adults in the summer of 2016 (July 2018)

Moving slightly out of the standardized testing realm, this month a team of Harvard researchers published the results of a study that monitored the cognitive performance of a group of young adults during a heat wave in Boston in summer 2016.

Similar to the studies previously mentioned, the researchers found that as the mercury rose, the subjects' performance fell on a battery of self-administered tests involving attention, cognitive speed and basic mathematical skills. The plot above, for instance, shows performance on what’s known as a Stroop test, which asks subjects to parse the difference between incongruently labeled color/word pairs. The warmer it got indoors, the harder that task became.

Dividing the subjects by whether they had air conditioning in their building of residence, the researchers found that a lack of AC was associated with a performance decline between 4.1 and 13.4 percent, depending on the test. Because the subjects were all healthy, young, college-age individuals, the authors noted that there’s a real possibility that deficits could be even greater among less healthy or less educated groups.

Study No. 4: Hot Temperature and High Stakes Exams: Evidence from New York City Public Schools (March 2018)

Back to the classroom: A March working paper by Jisung Park of Harvard University and the University of California at Los Angeles examined the effects of outdoor air temperature on 1 million New York City public high school students' performance on the New York State Regents examinations, a standardized test required to graduate from high school.

Again, the effects are large: Park found that “taking an exam on a 90°F day reduces performance by 14 percent of a standard deviation relative to a more optimal 72°F day.” To put that in context, that’s more than half the size of the within-school black-white achievement gap on the tests, which works out to about 25 percent of a standard deviation.

In Park’s sample, nearly 1 in 5 students experienced temperatures of 90 degrees or greater on exam day.

Because the exams are a prerequisite for graduation, heat exposure during the exams also has a direct effect on students' odds of graduating. “For the median student, taking an exam on a 90°F day leads to a 10.9% lower likelihood of passing a particular subject (e.g. Algebra), which in turn affects probability of graduation,” Park writes.

Administering the tests during the winter or spring, rather than at the end of the school year, would be one way to lessen the effect of heat on students’ scores. Air conditioning is the other — but data collected by Park shows that 38 percent of New York City public schools lacked air conditioning equipment in 2012.