Split household income inputs 50/50 between spouses in Microsim runner

[PavelMakarchuk]

Summary

Fixes #665. Fixes #838.

Splits household-aggregate TAXSIM income inputs between spouses in the PolicyEngineRunner Microsimulation path, matching the existing convention in input_mapper.py.

Allocation rules

Split 50/50 for MFJ, primary otherwise (6 types):

• intrec → taxable_interest_income
• dividends → qualified_dividend_income
• ltcg → long_term_capital_gains
• stcg → short_term_capital_gains
• gssi → social_security_retirement
• scorp → partnership_s_corp_income

Age-aware split for pensions (pensions → taxable_private_pension_income):

• Both spouses age 60+: split 50/50 (both qualify for per-person state pension exclusions)
• Mixed-age or both under 60: all to primary (avoids losing per-person exclusion in states like GA, MD, NJ)

The 60 threshold is the lowest common age across state pension rules (DE starts at 60; GA 62; MD 65).

Validation

612 records across all 51 states × 12 scenarios:

Metric	Result
Federal changed	0/612 (as expected)
Single filers changed	0/51 (no regression)
State improvements	22 records, +$5,853
State regressions vs TAXSIM	6 records, −$570 (all understood)
Net	+$5,282

Remaining small regressions (all understood)

All 6 regressions share a pattern: PE gives a lower state tax than TAXSIM, because PE applies per-person exclusions more aggressively — i.e. more legally accurate, but further from the TAXSIM benchmark.

id	state	ages	income	Direction
41, 42	AR	45/45	ltcg/stcg	PE further below TAXSIM; AR joint bracket behavior
44, 46	AR	70/70	pension	PE exclusion uses both spouses; TAXSIM less aggressive
70	CO	70/70	pension	PE larger refund
560	VA	70/70	pension	PE further below TAXSIM

All regressions push PE to lower tax (favorable to taxpayer) and are small individually (max −$237). These aren't new bugs — they're PE correctly applying per-person state exclusions that TAXSIM applies less completely.

Example (#838 — DE elderly couple, $44k pension, 2 deps)

TaxAct confirms 50/50 split: $22,249 per spouse, each with $12,500 exclusion (total $25,000).

	Before fix	After fix	TAXSIM	TaxAct
State taxable income	$40,000	$25,500	$27,500	—
State tax (siitax)	$848	$276	$386	$0

Tests added

tests/test_spouse_income_splitting.py — 13 focused tests:

• Each of the 6 non-pension income types: verify 50/50 split for MFJ
• Single filers with household income: verify full amount stays on primary
• Pension with both 60+: verify 50/50 split
• Pension with mixed-age or both under 60: verify stays on primary
• End-to-end DE elderly 20Kswages 44Kpensions 2depx #838 scenario: guards against regression of the DE pension bug

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
policyengine-taxsim	Ready	Preview, Comment	Apr 20, 2026 1:27pm