Cash is very easy to hide. ...

Cash is very easy to hide. It's easy to hoard. It's easy to move, especially these large bills.
 Kenneth Rogoff

Quotes from the same author

If you didn't have employers able to pay off the books and off the record safely in cash, you wouldn't have illegal immigration on nearly the scale that we do.
 Kenneth Rogoff
We all use cash in our everyday life, but we don't use hundred-dollar bills. We're not using 500-euro notes. And yet these account for mountains of cash out there. I think they're being used in tax evasion and by criminals of all types.
 Kenneth Rogoff
Today, chess programs have become so good that even grandmasters sometimes struggle to understand the logic behind some of their moves.
 Kenneth Rogoff
The assertion that everyone benefits simultaneously from free trade is simply incorrect.
 Kenneth Rogoff
There has been a noticeable decline in the labor factor in all wealthy countries in the past 20 years. The rich are getting richer, but those at the lower end aren't moving ahead as quickly as the capitalists.
 Kenneth Rogoff