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It's really nice when a classmate stops by your house when you're sick to bring you notes from your class you missed — but not like, remember-it-for-the-rest-of-your-life nice. But that time you introduced yourself to the new kid and they cracked a joke about your hometown?

It's been years, and that memory still makes your blood boil. That's probably because our brains kind of like holding onto grudges — even though it's better for our health to let things go. Before we really get into this, let's note that we're not saying anger is bad or wrong — sometimes, it's downright righteous.

And you may be totally justified in holding a grudge. But you should know that it costs you something, too. Experts say that when you hold a grudge, you keep experiencing the same negative emotions you felt during the offending event.

And feeling and re-feeling those emotions isn't good for your health. Ruminating on past offenses has been shown to raise a person's heart rate, blood pressure, and cortisol levels — which, physiologically, is similar to chronic stress. And if those things stay high for too long, they can damage your immune system and essential organs like your heart.

And studies have found that people who bear grudges are more likely to have physical ailments like heart attacks and stomach ulcers. This is probably because your brain processes mental imagery a lot like it processes the real deal. Think of someone who's afraid of spiders, for instance.

If they're asked to imagine a spider, their heart will race and their muscles will tense, just like they would if a spider was actually in front of them. Well, it's the same with your grudge. If you keep re-living the memory of someone embarrassing you, you keep feeling the embarrassment and anger.

And if that's not bad enough, grudges can also bleed over into other relationships. Research has found that ruminating on anger increases the odds you'll lash out at others, even if they had nothing to do with what made you upset. So, if grudges are so bad for us, why do we hold onto them?

Well, we can probably thank evolution for that. We're not the only species that does this. Studies have noted that all sorts of species, from ravens and crows to octopuses, can remember a person who wronged them and exact revenge afterwards.

Though, exactly how long they can hold these grudges isn't clear. One biologist noted that even guppies seem to take revenge on other guppies that don't pull their weight. If one individual falls too far behind when the group is inspecting a predator, the fish in front will fall back to leave the slowpoke exposed.

And some experts think that's because revenge and forgiveness are both important for cooperation — and therefore, essential to social species. Specifically, revenge helps ensure people don't let others walk all over them. People could just be hostile in general to prevent being used, but then they'd never form the relationships they need to survive.

And sometimes, it's impossible or unsafe to retaliate to an offense in the moment. If that person has the capacity to hold a grudge, though, they could mentally “practice” their revenge until it's the right time to exact it. That's certainly one way to keep the scales balanced.

And evolution doesn't really care if a behavior is dark or cruel. If people who held grudges produced more offspring than people who didn't, the neurological tendency to hold grudges would spread through the population. And that could explain why it's so common — even though grudge-holding isn't so helpful now.

It also might explain why, to your brain, plotting revenge is a goal like any other. The same structures activate whether you're planning to get even or planning brunch. That includes parts like the left prefrontal cortex, the dorsal striatum, and the caudate nucleus, which all play big roles in the neural circuits for motivation and reward.

So yeah, in a weird way, it feels good to get even. But, as satisfying as it might be to hold a grudge long enough to act on it, it's still bad for your health overall. Your hurt and angry feelings aren't just echoes of something that happened long ago — they're hurting you right now.

And when you forgive, you spend less time in stress mode, and that protects your mental and physical health. Of course, it's not always easy to release those feelings. Forgiveness takes work — like, it might take giving up some pretty powerful negative emotions or reconciling with a person that you might not feel deserves it.

But, there are ways to unburden yourself, if that's what you want. Take the Stanford University Forgiveness Project, for example. For a study published in 2006, they performed a randomized trial of a six-week forgiveness intervention where they taught participants a lot of the science we just covered in this video.

They also ran through visualization and meditation exercises designed to help them check in with their own emotions and learn alternatives to angry rumination, along with reframing the transgression to build more empathy toward their offender. And it worked! The intervention reduced the participants' negative thoughts and feelings two to three times more effectively than a control group.

So if you are holding onto a grudge — even one that is very well deserved — you may want to consider letting it go. After all, forgiveness doesn't mean forgetting or approving of the offending act, or letting the bad actor back into your life. It means letting go of your anger and not letting someone else be responsible for your well-being.

Forgiveness is a brave, difficult thing to give, but it's better for you in the long run. Thanks for watching this episode of SciShow Psych! We hope you learned something.

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