ID 2025 mixed age joint return

[feenberg]

See http://taxsim.nber.org/out2psl/730

This 2025 Idaho couple has 50000 in interest income and I believe they should recieve 30,000 regular standard deduction plus 1600 additional deduction for the one elderly partner. That would leave taxable income of 12398. PE shows 16898 - what are the additional 4470 in deductions?

[MaxGhenis]

I checked this against clean policyengine-us upstream/main, and the Idaho calculation there is already correct.

For the reported 2025 mixed-age joint case, PE-US main returns:

• state_standard_deduction = 33,100
• id_deductions = 33,100
• id_taxable_income = 16,898
• id_income_tax = 85.62799

That matches Idaho's federal conformity rule in Idaho Code 63-3022(j)(1): Idaho adopts the federal standard deduction, so for 2025 MFJ with one aged filer the correct amount is the federal amount, not $30,000 + $1,600 = $31,600.

So this does not look like a current PE-US bug. If there is still a taxsim-side discrepancy to resolve here, it would be in the taxsim issue framing or downstream interpretation rather than in current PE-US standard-deduction logic.

[MaxGhenis]

Adding the source documentation for the closure:

Statute:

• Idaho Code § 63-3022(j)(1): https://legislature.idaho.gov/statutesrules/idstat/title63/t63ch30/sect63-3022/
• That subsection says Idaho allows "the standard deduction as defined in section 63 of the Internal Revenue Code" or itemized deductions under IRC § 63, at the taxpayer's option.

Official 2025 Idaho form packet:

• 2025 Form 40 instructions (EIN00046, rev. 03-02-2026): https://tax.idaho.gov/wp-content/uploads/forms/EIN00046/EIN00046_03-02-2026.pdf
• The packet states that Idaho conforms to the IRC as of January 1, 2026.
• The 2025 standard deduction worksheet in that packet shows a base MFJ standard deduction of $31,500, plus $1,600 for one filer age 65+, for a total of $33,100.

That is why the premise in this issue was off: for 2025 Idaho MFJ with one aged filer, the official Idaho sources point to a $33,100 standard deduction, not $31,600.