We probably all have at least one friend who is constantly negative.

For me, one person comes to mind. She was just negative about everything. If anything positive happened at work, she would instantly misinterpret it as negative. For example, the free lunch was not a kind gesture, it was because they over worked us. In her opinion, every other worker suffered from inefficiency. I really dreaded seeing her when I walked into the office. Listening to her multiple daily rants became so energy consuming—her negative energy certainly dampened my enthusiasm. I even caught myself mirroring some of her negative gestures.

No more! I had to tune her out.

But, why was this happening?

We are social animals. We crave interactions with others and cannot live in isolation. Social interactions can protect our health and psyche. But, are all social interactions beneficial? As my above scenario demonstrates: no, some interactions are actually harmful to our health.

Here are five explanations of why and how being around negative people harms us psychologically and biologically:

We generate thoughts that are consistent with the current mindset in our brain.

That is why riddles are so much fun. Riddles require that we abandon the common mindset and use a different one to solve them. Perception of reality, which depends on current mindset, is crucial for our mental health. Do we see the cup as half empty or half full?

Being around someone who constantly complains or is overly pessimistic makes it harder to engage an optimistic mindset to interpret the world around us.

Humans have brain cells called mirror neurons. These cells are crucial for empathy, literally putting our minds in the other person’s shoes. Have you ever noticed that when you hang out with someone for a long time, you start to talk like them (sometimes even pick up their accent) and adopt their peculiarities? Or if someone around you yawns, you also uncontrollably yawn?

Thank your mirror neurons for that. We mirror each other’s emotions too. When we are around negative people, our mirror neurons pick up on that. Eventually, we adopt an attitude like them (scary thought).

We are social beings and our brains are designed to facilitate that outside our awareness. When we see our loved ones in danger, our hearts and breathing patterns synchronize with them. A study at Denmark Aarhus University showed that when a related observer watches his relative/friend walk on hot coals, the two synchronize their heat rates. Heart rate is certainly increased when we are angry, but this study suggests that it also increases when we are around people who are angry. A habitual increase in heart rate can cause cardiovascular problems.

Have you ever noticed what happens if you share office space with another female? After years of working together, a female’s menstrual cycle syncs with her female co-workers. The reason is because of pheromones. These are hormones released in an individual (male or female), which affect the behaviors of another individual. This is also why males who stay with their pregnant partners also show changes in hormones such as a drop in testosterone levels.

Negative people have a different hormonal signature. Hanging out with them may also affect our hormonal system because of pheromones.

We have two brains: one that is effortful and one that is automatic and effortless. The effortful one is responsible for our decision to read this article, make plans and any intentional behaviors. The effortless one is responsible for all our automatic reactions. Arguably, it is the automatic brain that is responsible for many of our behaviors and sometime it embarrasses us.

Like that time, I meant to go to a meeting and kept thinking about my favorite ice cream, and found myself in front of the store. Or intending to dial a phone number, but accidentally dial the phone number of another person who’s been on our mind. The automatic brain is a storage area of habits, past experiences, but also other’s influence. So, when our automatic brain’s storage is cluttered by our friend’s negative thoughts, whether your like it or not, we will react accordingly.

Stay away or limit interactions with overly negative people. If you suspect that you are a negative person, then seek help, a better life awaits you.

Marwa Azab lectures for the psychology, human development and biology departments. Her interdisciplinary background in psychology and biology affords her the opportunity to speak on various topics. She has a masters in Counseling Psychology and a PhD in Biological Sciences with emphasis on Neuroscience. She is a writer, public speaker, and life coach. Check out her TEDx talk. You can send her a speaking invitation on facebook or Linkedin.