É«É«À²

We also create communities that reinforce our tendency to choose friends who are similar to ourselves.

For example, if our friends are overweight, we have a 45% likelihood of being overweight. If the friends of our friends are overweight, our probability of being overweight increases by 20%.

Researchers have also proven that happy people are often surrounded by a lot of happy people and unhappy people are surrounded by unhappy people. This can be a sign of friend selection and also of how emotions spread in networks.

The power of networks is also visible in an experiment performed by researchers that measured the spreading of friendly acts in a network. When people were treated positively, they were friendly to others as well. A chain reaction can mean that treatment of a person may depend on how two people that are complete strangers to him/her have treated each other.

Watch Christaki's talk .