Here’s a concrete example. Suppose I make an offer to Fluther that if anyone gives me $1 on Monday, I’ll “invest” it and give back $1.50 two weeks later. I have lots of lurve, so people trust me.
So on the first Monday, daloon and gailcalled give me $1 each. I have $2.
Next Monday, cprevite and augustlan give me $1 each. Now I have $4.
The Monday after that, I have to pay out. daloon and gailcalled get $1.50 each. They are so pleased with their investment that they each invest another $1, and gailcalled talks up my scheme to ben and andrew, and they each put in $1. So on this Monday I paid out $3 and took in $4, so I have $5 on hand.
The next Monday, I have to pay out again. $1.50 to each of cprevite and augustlan, who are so thrilled that they invest $2 each. They talk this up to other people, and johnpowell and shilolo each invest $1. So I paid out $3 and took in $6, so now I have $8 on hand.
The Monday after that – another round of payouts. $1.50 to each of gailcalled, daloon, ben, and andrew, who each give me another $1. And johnpowell is worried that other people are making money and he isn’t, so he invests another $3. $6 out, $7 in. Now I have $9 on hand.
You can see at this point that the profits that earlier investors see are coming from the investments of people who contribute money later.
Now, I’m in trouble. My fund is not solvent. In the past two weeks, I have $1 each from gailcalled, daloon, ben, andrew, and shilolo, $2 each from cprevite and augustlan, and $4 to johnpowell. This means that if they all want their money back, I have to pay them $13—but I only have $9 on hand. But as long as people keep on investing in greater and greater numbers, or with larger and larger amounts, I’m OK.
Now, suppose Flutherers have less disposable income, and decide to spend all their spare dollars on cake for their girlfriends. Nobody else is investing.
So on the next Monday, I have to pay $3 to each of cprevite and augustlan, and $1.50 to each of johnpowell and shilolo. Each of them are pleased with the outcome, and invest the same initial amount – $2 for each of cprevite and augustlan, $1 for each of johnpowell and shilolo. So I paid out $9 and took in $6. I have $6 on hand, and things are starting to look bad.
The Monday after that, I have to pay $1.50 to each of gailcalled, daloon, ben, and andrew, and $4.50 to johnpowell. Er, that’s $10.50, and I only have $6 on hand. Suddenly my checks bounce. And worse, I don’t have enough on hand to refund anyone’s initial investment.
Multiply these numbers by a million, and run it through a few more cycles, replacing names of Flutherers with names like “Brandeis University,” and you have what Madoff did.