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"The two things you need to know about economics"

By Tim Harford Worstall. #1 is "Incentives matter" and that's important and useful enough that it doesn't much matter what #2 is.

UPDATE: the author of the piece is Tim Worstall. I apologize to Mr. Worstall and thank him for the correction.

For much more on the "Two things" game see this collection. Examples:

The Two Things about Accounting: 
1. The trial balance must balance. 
2. There's a lot of "grey area." 

The Two Things about Computer Programming: 
1. Idiocy increases faster than idiot-proofing. 
2. All compiling errors boil down to a missing semicolon.

The Two Things about Star Trek: 
1. Don't beam down in a red shirt. 
2. You can always talk evil computers into destroying themselves. 

The Two Things about Writing: 
1.  Include what's necessary. 
2.  Leave everything else out.