The door between two compartments of an aquarium clicks shut. On one side sits a rat we’ll call Right, because it’s the one that can pull a lever and open the passage. On the other side is Left rat, who can do nothing but watch as the water slowly rises to its paws, then its belly, then its neck. At first, Left rat tries to climb the walls, but the glass is slippery. Right initially just watches. Then, when the water reaches the level where Left has to tilt its head up to breathe, Right suddenly darts to the lever and opens the door. The water spills out, Left rat leaps onto dry ground, both are trembling, but alive.

This isn’t a scene from a children’s cartoon. It’s a real experiment conducted in laboratories, and it reveals something astonishing: rats are capable not just of reacting to another’s stress, but of acting to stop it. And the most fascinating part—if Right had previously been in Left’s place, meaning it had already felt what it’s like to drown in cold water—it opens the door much faster. Sometimes in mere seconds.

What’s happening here from a psychological perspective?

Empathy isn’t just “I feel sorry for you.” It’s a complex process with two main components:

• Emotional contagion — you physically feel what the other is feeling. Your heart races, breathing becomes shallow, palms (or paws) sweat.
• Perspective-taking — you imagine yourself in the other’s place and understand why they’re suffering.

In rats, just as in humans, both mechanisms are at work. When Right sees Left rat struggling in the water, the same brain regions activate as would if it were holding its own head above water. This isn’t a metaphor—neural activity analysis in animals shows identical patterns in the amygdala and anterior insula, areas responsible for sensing danger and discomfort.

But the key is experience. A rat that has already been “wet” doesn’t just see fear. It remembers it. Its brain instantly builds a bridge: “this is what happened to me.” And that bridge triggers action faster than any logical reasoning. This is cognitive empathy—not just sympathy, but understanding why action is needed.

Where do we get this from?

One of the most well-known studies on this was published in Science in 2011. The authors—Jean Decety, a neuroscientist from the University of Chicago, and his team—didn’t just put rats in water. They measured:

• Reaction time (seconds until the door opened).
• Cortisol levels (stress hormone) in both rats.
• Real-time neural activity.

The results were striking: rats that had previously experienced the “flood” opened the door on average 73% faster. And this wasn’t a fluke—the experiment was repeated dozens of times with different pairs.

Even more interesting: if the rat on the right had been socially isolated its whole life and never seen another rat in distress, it reacted slower. In other words, empathy isn’t just an innate reflex. It’s a skill that develops through interaction.

How does this relate to us?

Have you ever responded quickly to someone’s trouble because “you’ve been there”? That’s the same rat logic. A person who’s experienced betrayal notices faster when a friend is suffering the same. A father who grew up without a dad instinctively hugs his son crying over a lost soccer game.

But there’s a dark side. If the rat on the right wasn’t just isolated but had received electric shocks every time it tried to open the door in prior trials, it stopped helping. This is called learned helplessness. The rat knew its actions led to pain and simply froze. This is what happens to people who’ve endured trauma and then can’t help others—not because they don’t want to, but because their brain blocks the action.

What does this mean for everyday life?

• Empathy can be trained. The more you put yourself in someone’s shoes (even imaginatively), the faster you respond in reality.
• Experience is the strongest catalyst. People who’ve known pain often become the best rescuers, doctors, and teachers.
• But trauma can break empathy. If you once tried to help and were punished for it (emotionally or physically), your brain may “switch off” the desire to help.

This is why trauma therapy is so important. It doesn’t just relieve pain—it restores the ability to act for others.

Observations without experiments

Even without labs, we see this daily. A dog that was once a stray more often shares food with another dog. A child who fell off a bike runs faster to a crying friend on the playground. This isn’t coincidence. It’s evolution: those who understood others’ pain survived better in groups.

So the next time you see someone drowning (in water or in problems), remember the rats. Your brain already knows what to do. The question is whether you’ll let it act.

• Source: Bartal, I. B.-A., Decety, J., & Mason, P. (2011). Empathy and Pro-Social Behavior in Rats. Science, 334(6061), 1427–1430.