> But if banks wanted, they could actually track the serial codes on the bills around. And since often cash is used in only 1 hop (so ATM -> you -> shop -> bank

> But if banks wanted, they could actually track the serial codes on the bills around. And since often cash is used in only 1 hop (so ATM -> you -> shop -> bank) they could know pretty well where you spent your money. That doesn't include how much you spend exactly. And it only works most of the time (what if my money gets used as change for another customer?).