[jtheory]

There's a big debate about this, though it's a tangent to this discussion. But it boils down to how adding money into the equation tweaks the incentives of those giving blood or donating organs in a way that's not necessarily good.

One obvious way -- suppose you're filling out a medical history form before providing blood or an organ, and you know that ticking "yes" to a question will disqualify you. You really need the money -- will you be honest?