3 degrees of contagion – how feelings spread

cartoon of three characters in a circle, one green, one blue and one yellow with arrows going around the circle from one to another.
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A lot of things are contagious. We’re all familiar with the word from our experience of Covid. We always think of it in terms of viruses, bacteria, or some other pathogen. 

But lots of things are contagious. Happiness is contagious. So is depression. Loneliness is too. And so also are lifestyle habits and behaviours that lead to changes in our weight. My favourite is kindness. It is highly contagious.

Happiness is contagious due to the Mirror Neuron System (MNS). It’s a network of brain cells that mirror the actions we observe in others. It’s highly active when you spend time with someone.

If the person happens to be happy at that time, your MNS mirrors the activity of their facial muscles that express their happiness. 

The word ‘mirror’ comes from the fact that your MNS really does mirror these muscle movements because it reproduces the same ones in you. The more they smile, the more you smile as if you were trying to copy them, only you do it involuntarily. It’s an unconscious process.

But your MNS does more than that. It not only mirrors their facial muscles, but also the emotional state that’s consistent with those facial muscle movements. And thus you ‘catch’ happiness from happy people. Negative emotions are contagious in this way too.

The amazing bit is just how far you can infect people with your emotions and how people you don’t even have any contact with can infect you with theirs. Research shows that happiness (and depression) is contagious to three degrees of separation.

It means that if your sister’s, neighbour’s best friend felt happier for some reason, her gain in happiness might actually ripple over to you and make you feel a boost. You’ll have absolutely no idea where the source of your positive mood originated from.

Your sister is one degree of separation from you; she’s an immediate close contact. Her neighbour is two degrees of separation from you. She’s not your contact or she would be one degree from you. She’s your sister’s contact, so she’s two degrees away. That makes her neighbour’s best friend three degrees of separation from you.

It’s the same principle as the six degrees of Kevin Bacon game and the fact that there’s no more than six degrees of separation between you and any other person on the planet. In other words, you know someone (1) who knows someone (2) who knows someone (3) who knows someone (4) who knows someone (5) who knows HH the Dalai Lama personally, or is friends with King Charles III, or knows the actress Meryl Streep.

If you want some numbers, analysing the data from a large social network of over 12,000 people, James H. Fowler, professor of medical genetics at UC San Diego, and Nicholas Christakis, professor of social and natural science at Yale, found that there would be a 6% chance of you feeling happier simply because your sister’s, neighbour’s best friend became happier. That’s even though you have never met your sister’s, neighbour’s best friend and you most likely never will.

The chances of you becoming happier are about 10% at two degrees of separation (your sister’s neighbour) and 15% at one degree (your sister). If you have a very good relationship with your sister, though, and see her often, then the chances of you feeling happier because she is feeling happier goes up to 63%. 

Of course, these numbers are averages over a huge number of social connections and will vary with each person, but they offer are a good rule of thumb that demonstrate just how much of a ripple effect our emotions have.

What about other things?

Well, Fowler and Christakis showed that depression is also contagious to three degrees of separation. Loneliness is too. In fact, the stats show that you have just about as much chance (6%) of catching loneliness as happiness.

How on Earth? 

I wondered the same thing when I was first researching the subject. I covered it all in a book, The Contagious Power of Thinking, that I wrote back in 2011.

The contagion of loneliness is through social contagion rather than emotional contagion, which is how happiness and depression mostly transmit. Here’s an idea of how it can work.

Say you are friends with someone who has just lost their job or experienced a relationship breakup. Even though they still have you as a friend, there’s a net downwards change in how they feel. They now have less immediate close contacts than they had before. They feel a little more lonely because they have lost significant people (colleagues or a partner) from their social network.

This change in how they feel causes them to withdraw a little in life, staying in more, isolating themselves from life a bit. This has an immediate effect on you because you rely on that person as one of your key friends. It hits you and affects your social life. Now you’re feeling more lonely because your number of connections and interactions is down.

Despite the fact you have other friends, you still feel the net downward change. You’re only human. It causes you to hold back a bit, isolating yourself more. This has a net effect on some of your friends, who rely on your friendship. If you have three friends, at least one of them probably relies on you enough to now feel lonelier.

And so your friend losing his job has now caused one of your friends, whom your first friend doesn’t even know, to feel lonely. And so it goes on.

Of course, this is just one of a vast number of different ways that relationships can change, and a person can feel lonely for any number of different reasons, but it illustrates an example of how feelings and behaviours can ‘ripple’ through social networks.

Network studies show that weight gain is also contagious to three degrees of separation. Weight loss works in much the same way. In fact, your sister’s (1 degree) boss’s (2 degrees) best friend (3 degrees) can make you lose weight.

Your sister’s boss’s best friend has been studying nutrition lately and has embarked on eating healthier and exercising more. Her behaviour rubs off on her best friend (your sister’s boss) who starts eating better and exercising more frequently too. Within a few weeks she’s feeling healthier and has lost a little weight as a consequence.

During a conversation, your sister compliments her boss on how well she’s looking. Her boss enthusiastically explains her recent healthy lifestyle change.

Your sister is quite taken by the notable change in her boss and decides to make some lifestyle changes herself. Within a few weeks she’s also feeling better and has lost some weight.

You wonder at your sister’s change. Not only is she looking different but she’s feeling better too and seems to have more energy. You can’t help but ask what she’s been doing. 

She tells you of her increased consumptions of fruits, veggies, and salads, and that she’s making smoothies, and has started going to classes at her local gym. She cooks you a healthy meal and it’s amazing. It lights up your taste buds. You had no idea healthy food could taste so good.

You’re impressed and before long you’re joining her in cooking up healthy meals. You’ve started jogging too.

You thank your sister for having such a positive effect on you, but actually her change originated in the change from her boss’s best friend who is at three degrees of separation from you. You’ve never met her and you probably never will. Yet she is kinda responsible for your better health. You caught a ‘healthy bug’ from her.

Other things are contagious too. Alcohol consumption, giving up smoking. Even divorce is contagious, but only to 2 degrees of separation.

One of the most contagious things of all is kindness. Like emotions and behaviours, kindness is also contagious to three degrees of separation. And when you’re kind to someone, chances are they won’t just pass on kindness to one other person, but to several.

The R-number for kindness might be somewhere between 3 and 5, meaning one act of kindness might be passed on to 3 to 5 people. Let’s use 5 as an example to make the point.

Suppose you do something kind for someone. On account of how you made that person feel, she or he will likely be kind or kinder to five people. Those 5 people are at one degree of separation from you.

But each of those 5 will also be kind or kinder to 5 people and so 25 people will receive some kindness at 2 degrees of separation from you. And it continues. Each of those 25 will also be kind or kinder to 5 people and so 125 people will benefit from some kindness at three degrees of separation from you.

Most of these people you will never meet in your life and yet they are wearing a smile on their face today as a consequence of the ripple effect of the kind thing that you said or did a few days ago.

It’s like dropping a pebble in a pond. The waves move out in a circle. Lily pads rise and fall at the far sides of the pond with no idea of why they’re being buoyed. But it’s because of the wave, which you set in motion by dropping the pebble in the pond.

Every day, we drop these pebbles of kindness through the things we say and do. Most times we hardly give a thought for the consequence of our actions, but be rest assured that you’re lifting lily pads everywhere.

Don’t ever underestimate how much of an impact you have on people or the wider community by being kind.

One small kind word or act from you can be significant for someone else.


  1. Renate on September 23, 2022 at 9:26 am

    Thank you David, with a bright smile from Southgermany ☀️Renate
    I am so grateful for your newsletter and happy to receive them

    • David Hamilton on September 23, 2022 at 10:47 am

      Aw, thanks for your kind words, Renate. 🙂

  2. Laura Hope Cordell on September 23, 2022 at 11:14 am

    I love this. It shows that it’s easier than we think to achieve our mission of spreading kindness around the world. We have 3700 people in our Kindness Community http://www.facebook.com/groups/kindcommunity so in theory if they each do an act of kindness 462,500 could benefit. Amazing!

  3. Val on September 23, 2022 at 1:21 pm

    Thanks David, always appreciate the insight and explanation your work brings. This is a great reminder of how connected we all are and while I have been aware of how contagious kindness and happiness can be, I have never given a thought to how this can apply to alcohol consumption, weight etc. Love the expansion.

  4. Monika Ashton on September 23, 2022 at 4:11 pm

    Every day you make a choice of being kind or not. Think of being in a grocery store and letting the person with one or two things go ahead of you. Sharing a laugh with someone. After a while it becomes a way of life.
    That is how I see it. Choose to be kind it helps every situation. I loved your article.

  5. Dagmar Siivonen on September 23, 2022 at 4:44 pm

    Hello David ! I write to you from Finland ! I am glad to have come into contact with you “by accident” through an interview with someone in Youtube. Your so kind and warm radiation touched me immediately. Here in my beloved country of Finland kindness is especially necessairy. Perhaps because the climate has a long dark and cold phase, people here are often not so social and outgoing and friendly, especially to “strangers”. I have learned to smile on the streets to even unknown people, and the response has been so good.
    Love from Dagmar

  6. Monika Ashton on September 23, 2022 at 4:59 pm

    Being thoughtful .

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