This is Origin Stories, the Leaky Foundation podcast. I'm Meredith Johnson. You've probably done something nice for someone else today without even thinking about it.

I think we've all had the experience of people, strangers, stepping up to help us when they didn't need to.

You know, from holding a door to jumping out of their car when you've had a car accident to see what they could do,

to, you know, bring it back your dog that's gotten lost. We've all had that experience. We do these kinds of things, sometimes

at great cost, or even great risk to ourselves. Why? Our species' strong tendency towards helping

others has been a puzzle to scientists since Darwin. He first considered our self-sacrificing

ways in his 1871 book, The Descent of Man, where he wrote about it in problematic Victorian language,

saying, quote, he who is ready to sacrifice his life, as many a savage has been, would often leave

no offspring to inherit his noble nature.

Darwin worried that altruism was the one weird behavior that might mess up the whole idea of natural selection.

After all, if natural selection is about surviving to reproduce and making sure your own genes are carried on,

why would people ever do things like rush into a burning building to save a stranger?

Or on a more day-to-day level, why would we give money to charity,

or give up food that could go to ourselves or our own kids?

Why would natural selection favor behaviors that seem to make us less likely to survive?

It's a big question for biologists.

When biologists use the term altruism, they mean very specifically

behaviors that are costly to the individual who performs the behavior

and beneficial to the individual who is the recipient of the behavior.

So if I have a pizza and I give you half my pizza,

that's costly to me because I've now given up half my pizza,

but beneficial to you because you now have half a pizza.

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