Welcome to the Saturday edition of The Conversation U.S.’s Daily newsletter.

Why do people have friends? If you watch little kids on the playground, you can see how natural it is to play with a pal who’s having fun alongside you. Sharing a laugh, trying something new together, reminiscing about past escapades – there are lots of reasons to participate in a friendship.

Psychology researchers Jessica Ayers, at Boise State, and Athena Aktipis, at Arizona State, write that their field doesn’t have one agreed-upon definition for what friendship is. A traditional assumption has been that friendship is a transactional relationship – participants keep score with an eye on, “What have you done for me lately?” People drop friends who take more than they give.

But that doesn’t feel right to many people. As “anybody who has seen a friend through tough times – or been the one who was supported – can tell you,” Ayers and Aktipis write, “keeping track of what a friend does for you isn’t what friendships are about.” Their research across cultures has identified the way friends can be like “social insurance systems for each other, helping each other when needs arise because of unpredictable and uncontrollable events.” No one expects to get paid back in these cases. And this risk-pooling model may be one aspect of how our ancient ancestors survived.

This week we also liked stories that explain the science and significance of this year’s Nobel Prizes, the rise of independent reporters to break big stories and the history of religion and labor rights in coal country.

One last note: If you find our work valuable, please support us. We’re giving all our donors a free e-book of our recent series looking at bold solutions to the affordable housing crisis.