diff --git a/changelog.d/368.md b/changelog.d/368.md
new file mode 100644
index 000000000..3d6318809
--- /dev/null
+++ b/changelog.d/368.md
@@ -0,0 +1 @@
+- Set Marriage Allowance take-up rate to 0.5 (HMRC outturn ~2.1m claimants of ~4.2m eligible couples) instead of the placeholder 1.0, so microsimulation no longer overstates Marriage Allowance cost by ~£500m/year.
diff --git a/policyengine_uk_data/parameters/take_up/marriage_allowance.yaml b/policyengine_uk_data/parameters/take_up/marriage_allowance.yaml
index ff528bba7..603c4b4bd 100644
--- a/policyengine_uk_data/parameters/take_up/marriage_allowance.yaml
+++ b/policyengine_uk_data/parameters/take_up/marriage_allowance.yaml
@@ -1,6 +1,9 @@
-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 (~50% take-up). Read by `datasets/frs.py` to populate `would_claim_marriage_allowance` stochastically per person.
 metadata:
   unit: /1
   label: Marriage Allowance take-up rate
+  reference:
+    - title: House of Commons Library — Income tax allowances for married couples (SN00870)
+      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
 values:
-  2000-01-01: 1
+  2000-01-01: 0.5