>> So if for example I know that my uncle is not paying all his taxes I'm a criminal too unless I report him? Or is it only for bigger frauds?

> Yes if the government can prove that you knew about it without telling, you can be charged with it.

That's not generally true. In the vast majority of cases, simply knowing about a crime and not reporting it is not a crime in itself. You only become liable when you aid-and-abet or join in a conspiracy to commit the crime.

However, it depends very much on the specific crime. Tax evasion may be entirely different than say selling drugs.

The U.S. revenue-side of government isn't encouraging you to start illegal businesses to improve tax receipts, they probably get almost no taxes from that form-box ... it's just much easier later when authorities find out about your illegal activities to convict you of evading taxes that were clearly and easily payable in the past than to convict you of your particular illegal behavior.

Also the case in Canada, to my knowledge. A few years ago, a TV show called the tax agency anonymously, claiming to be drug dealers and asking how to report their earnings. The tax agency person was helpful and cooperative, and said they can't report it to police.