diff --git a/changelog.d/12859.feature b/changelog.d/12859.feature
new file mode 100644
index 000000000000..e674c31ae8aa
--- /dev/null
+++ b/changelog.d/12859.feature
@@ -0,0 +1 @@
+Experimental support for [MSC3772](https://github.com/matrix-org/matrix-spec-proposals/pull/3772): Push rule for mutually related events.
diff --git a/synapse/storage/databases/main/relations.py b/synapse/storage/databases/main/relations.py
index 3b1b2ce6cb8d..b457bc189eb4 100644
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@@ -13,7 +13,6 @@
 # limitations under the License.
 
 import logging
-from collections import defaultdict
 from typing import (
     Collection,
     Dict,
@@ -810,7 +809,9 @@ def _get_event_relations(
             txn: LoggingTransaction,
         ) -> Dict[str, Set[Tuple[str, str]]]:
             txn.execute(sql, [event_id] + rel_type_args)
-            result = defaultdict(set)
+            result: Dict[str, Set[Tuple[str, str]]] = {
+                rel_type: set() for rel_type in relation_types
+            }
             for rel_type, sender, type in txn.fetchall():
                 result[rel_type].add((sender, type))
             return result
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index eda92d864dea..867f315b2ace 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -595,13 +595,14 @@ def cached(
 def cachedList(
     *, cached_method_name: str, list_name: str, num_args: Optional[int] = None
 ) -> Callable[[F], _CachedFunction[F]]:
-    """Creates a descriptor that wraps a function in a `CacheListDescriptor`.
+    """Creates a descriptor that wraps a function in a `DeferredCacheListDescriptor`.
 
-    Used to do batch lookups for an already created cache. A single argument
+    Used to do batch lookups for an already created cache. One of the arguments
     is specified as a list that is iterated through to lookup keys in the
     original cache. A new tuple consisting of the (deduplicated) keys that weren't in
-    the cache gets passed to the original function, the result of which is stored in the
-    cache.
+    the cache gets passed to the original function, which is expected to results
+    in a map of key to value for each passed value. THe new results are stored in the
+    original cache. Note that any missing values are cached as None.
 
     Args:
         cached_method_name: The name of the single-item lookup method.
@@ -614,11 +615,11 @@ def cachedList(
     Example:
 
         class Example:
-            @cached(num_args=2)
-            def do_something(self, first_arg):
+            @cached()
+            def do_something(self, first_arg, second_arg):
                 ...
 
-            @cachedList(do_something.cache, list_name="second_args", num_args=2)
+            @cachedList(cached_method_name="do_something", list_name="second_args")
             def batch_do_something(self, first_arg, second_args):
                 ...
     """