Why does it matter whose W-2 it is?

[Condo2d]

Married Filing Jointly.  Three W-2's: 1 for me, 2 for spouse

TurboTax mistakenly identified one of our W-2's as mine.  When I corrected the name on it in TurboTax, the 'tax due' was reduced by $200?  What?!

Why does it matter whose it is?  I thought MFJ just added all the incomes together and calculated taxes based on the combined income.  I didn't change anything except the dropdown to select that it belonged to my spouse.

[VolvoGirl]

Yes it matters a lot.  There are several reasons.  I can't think of them all but a big reason is if one person went over the Social Security max and has excess SS to be refunded.  If one person had more than 1 employer and paid more than $9,114.00 in Social Security tax (box 4) combined you get the excess back on your tax return.  It's for each spouse separately.

And it can affect any IRA contributions limits.  

[BillM223]

There are certain situations where it matters. For example, for the Child and Dependent Care Credit, both spouses must have earned income to get the credit, and when both W-2s are accidentally assigned to the same spouse, then the result is often no credit.

I am sure that there are other similar situations. You are correct that the tax calculation is based on adding all W-2s together no matter who they belong to, but there can be other more subtle issues that affect your tax outcome.

[Condo2d]

Great point!

I'm sure it bumped me over that SSN withholding limit.  Thanks.