Reflecting on feelings of anger can cause physical changes in how the body processes the emotion, according to a new study by researchers at Carnegie Mellon University and the University of California, San Francisco.

One basic law of science is that the act of measuring a thing can alter it—a camera’s lens will distort the light that passes through it, a thermometer built to measure absolute zero will generate traces of heat, and a teenager is more likely to lie when his parents are watching. And it turns out that reflecting on feelings of anger actually changes the body’s physical response to the emotion.

Many studies involve asking subjects to self-report their emotions. Dr. Karim Kassam and Dr. Wendy Mendes, in a study published today in PLOS ONE, wanted to understand more about how the act of self-reporting can affect a subject’s emotional and physical state.

“Wendy and I do a lot of emotion research,” said Kassam, an assistant professor of social and decision sciences at Carnegie Mellon University. “How can you know what someone is feeling unless you ask people? But does asking people how they’re feeling change how they’re feeling?”

In their study, Kassam and Mendes had their subjects perform a difficult mathematics task. Some of the subjects received negative feedback on their performance from an experimenter, while others did not.

Of those who received the negative feedback, some got feedback designed to cause feelings of anger—the experimenter behaved rudely and incompetently, for example—while others received feedback that suggested the subject’s poor performance was his or her own fault, which was meant to cause feelings of shame.

After the test and feedback, some of the subjects were asked to report how they felt, while others were not. Through the whole experiment, Kassam and Mendes measured the subjects’ vital signs to see whether their bodies’ fight-or-flight response system was activated.

Their results were striking. Unsurprisingly, feelings of shame and anger provoked a greater physical response than neutral feelings, though the response to anger was more extreme. The difference between anger and shame was apparent after the subjects offered a self-report. While reflecting on shame had no particular effect, reflecting on anger completely changed the subjects’ physiological responses.

On its own, anger causes a challenge response—an activation of the fight-or-flight system. Heart rate increases and blood flows from the brain and central organs out to major muscle groups, preparing you to face off against a saber-toothed tiger. But when subjects reflected on their anger, they instead showed a threat response—imagine a deer freezing in the headlights of an oncoming car—also known as a fright response, with lower heart rate and blood concentrating in the core of the body.

So why is anger so different from shame?

“Shame is a self-conscious emotion that people are aware of, whereas with anger, people may not be thinking about that,” Kassam told Healthline. “People can go through aspects of their lives and not really think about how they’re stressed or angry. It’s in the back of their mind. Asking them to think about it helps bring it to the forefront.”

Although talking about feelings of anger does lower heart rate and blood pressure, Kassam warns that this is not always a solution. The challenge response could then be replaced with a threat response, which may not be a wise trade-off.

“What we see in terms of a cardiovascular response is that it’s worse when it’s brought to the forefront,” Kassam explained. Repeat activation of your body’s threat response can end up causing chronic stress and depression. “If you’re ruminating on your anger in a situation where you cannot extract yourself from it, awareness may not be a good thing.”