How do you incentivize people to provide missing info if it will increase their tax bill?

The US system is intentionally designed so that you don't know what the IRS does or does not know. If you leave something off of your taxes that the IRS knows about then maybe they'll audit you and give you a big fine. So the optimal strategy is to report everything and pay the taxes you owe.

In Spain's system, it seems to me that the optimal strategy would be to either go with what the tax agency says or to report more information, depending in which gives you a lower tax bill.