It's are because the phrase economics is used as a plural term in the following sentence.
It will follow a general noun and is not set off by commas is true regarding a restrictive adjectival clause. Since this clause is restrictive, it means it is essential for the meaning of the noun and cannot be excluded.
Idioms exist in every language. They are words or phrases that aren't meant to be taken literally. For example, if you say someone has "cold feet," it doesn't mean their toes are actually cold. Rather, it means they're nervous about something. Idioms can't be deduced merely by studying the words in the phrase.
