I just realized something concerning who should file taxes?

It's the REQUIREMENT to file vs the advisability to do so.

A married couple with 2 kids and an income of $18,000 is not REQUIRED to file. It would be highly advisable for them to do so as they would forgo thousands in EIC and the Additional Child Tax Credit in addition to the Making Work Pay Credit if they didn't.

This is nothing new. I've been advising hundreds of folks for over 40 years that they SHOULD file even when they are not legally REQUIRED to when it's in their best interest do do so. Even newbie tax preparers quickly figure this out.

You're right, anyone who can get a refund should file even if they aren't legally required to file. That means anyone below the filing requirement who had taxes withheld, or anyone who qualifies for a refundable credit such as the EIC or MWP credit. (The "Additional Child Tax Credit" is also refundable).

As Iffy pointed out, most of the people who only make a few hundred dollars are teenagers who work part time and are still claimed as dependents on their parent's returns. Dependents never qualify for refundable credits, so they would only have a reason to file if they had taxes withheld. If they were smart they would have put "Exempt" on their W-4 and had no taxes withheld in the first place.

If your income is below $x, you are not required to file. That is it is not mandatory. However, you should file to get refund.

Yes, if you have earned income you get Making Work Pay credit. Each years IRS gets billions of dollars as many people who have refund do not file.

Good point. Although in addition to iffy's valid comment, a lot of the rest of the people are getting EIC so would file anyway.

teenagers working part time don't get the making work pay since they are claimed as dependents on someone else's tax return.