Hello and welcome to Planet Money Summer School, the only educational institution in the

world that isn't freaking out right now.

This is class number four, scarcity and pistachios.

I'm Robert Smith.

Every Wednesday till Labor Day, we are teaching you to look at the world through economic

eyes.

And we're halfway done.

Don't worry, no midterm, but there will be a final exam and a high quality certificate

at the end that just might, in certain lighting conditions, be mistaken for a diploma.

You know who else is fancy diplomas?

Betsy Stevenson and Justin Wolffers from the University of Michigan.

Hi, how are you today?

I'm so excited for this episode.

You say that for every episode.

They're all about economics.

I feel like our first three classes were all about making individual decisions to maximize

fun and profit.

We looked at cost benefit analyses, markets and prices, and it was all nice and orderly.

But this is the part of the course where things are going to get messy.

Yeah, the world's messy.

Look economics isn't just about you making your decision.

It's about how people interact.

...