[doldols]

Of course, but the fifth amendment only protects you from self-incrimination if you explicitly invoke it (or keep your mouth shut). Not if you lie.

[saghm]

Does this mean you could fill in illegal income in your taxes and then for the source put down "I plead the 5th"?

[Bilal_io]

Then that would give them a reason to investigate.

They cannot ask you what you're pleading the 5th for, but they can go investigate it, and nail you when and if they find proof of illegal activity.