PolicyEngine · MaxGhenis · Apr 20, 2026 · Apr 20, 2026
diff --git a/changelog.d/623.md b/changelog.d/623.md
@@ -0,0 +1 @@
+- Set Marriage Allowance take-up rate parameter to 0.5 (HMRC outturn ~2.1m claimants of ~4.2m eligible couples) instead of the placeholder 1.0, with a House of Commons Library citation. The parallel rate in `policyengine-uk-data` is the load-bearing one for microsimulation outputs and must be updated separately.
diff --git a/...cyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml b/...cyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml
@@ -1,9 +1,11 @@
-description: Percentage of eligible couples who claim Marriage Allowance.
+description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible couples (~50% take-up). The matching parameter in policyengine-uk-data (`parameters/take_up/marriage_allowance.yaml`) is the load-bearing one for microsimulation, applied stochastically when populating `would_claim_marriage_allowance` per person; this file should be kept in sync.
 values:
-  2000-01-01: 1
+  2000-01-01: 0.5
 metadata:
   unit: /1
   label: Marriage Allowance take-up rate
   reference:
     - title: Income Tax Act 2007 s. 55B
-      href: https://www.legislation.gov.uk/ukpga/2007/3/section/55B
+      href: https://www.legislation.gov.uk/ukpga/2007/3/section/55B
+    - title: House of Commons Library — Income tax allowances for married couples (SN00870)
+      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		- Set Marriage Allowance take-up rate parameter to 0.5 (HMRC outturn ~2.1m claimants of ~4.2m eligible couples) instead of the placeholder 1.0, with a House of Commons Library citation. The parallel rate in `policyengine-uk-data` is the load-bearing one for microsimulation outputs and must be updated separately.