A July survey from Finder.com found that 69.8 million of the Americans making good money working side hustles aren't declaring that additional income on their taxes.
That's a fourth of all the Americans participating in the gig economy.
While millennials are the savviest and most enterprising of all the generations, bringing in more than the others with an average of $3,677 a year, they're also the worst at reporting that income. The report found that 33 percent don't declare it on their taxes.
Meanwhile, 26 percent of Generation Xers and 17 percent of baby boomers aren't being fully honest, either.
The IRS says tax evasion costs the government $458 billion a year. Of that total, according to the report, $214.6 billion is estimated to be from undeclared side-hustle income.
The short-term, flexible jobs of the gig economy make this possible. One of the most common is babysitting, for example. Sitters are usually paid in cash.