26 USC 7206:
Any person who— (1) Willfully makes and subscribes any return, statement, or other document, which contains or is verified by a written declaration that it is made under the penalties of perjury, and which he does not believe to be true and correct as to every material matter [...] shall be guilty of a felony and, upon conviction thereof, shall be fined not more than $100,000 ($500,000 in the case of a corporation), or imprisoned not more than 3 years, or both, together with the costs of prosecution.
("Return", as defined in 26 USC 6696, includes an income tax return.)
This clause notably doesn't care whose favor the falsification was in, or why you did it. You knew it wasn't true and you put it on your tax return anyway, and you declared under penalty of perjury that it was true and correct. That's a felony, end of story.
(Paragraph (4) of this section covers some other types of fraud done "with intent to evade" taxes, but that restriction does not apply to paragraph (1).)
The likelihood of actually being prosecuted in such an instance is beyond the scope of this site. But in the given case, the falsification really isn't in the government's favor. The purpose was to become eligible to collect government social security benefits, which you otherwise wouldn't be eligible for. Presumably you would only bother to do this if you thought you'd collect more in benefits than you pay in extra tax, which means the government loses in the long run.
This also eliminates any chance you'd have of claiming the falsification was "immaterial": it affected your eligibility for social security, and you knew that - it was the whole reason you did it.