diff --git a/.idea/.gitignore b/.idea/.gitignore
new file mode 100644
index 000000000..26d33521a
--- /dev/null
+++ b/.idea/.gitignore
@@ -0,0 +1,3 @@
+# Default ignored files
+/shelf/
+/workspace.xml
diff --git a/.idea/inspectionProfiles/profiles_settings.xml b/.idea/inspectionProfiles/profiles_settings.xml
new file mode 100644
index 000000000..105ce2da2
--- /dev/null
+++ b/.idea/inspectionProfiles/profiles_settings.xml
@@ -0,0 +1,6 @@
+<component name="InspectionProjectProfileManager">
+  <settings>
+    <option name="USE_PROJECT_PROFILE" value="false" />
+    <version value="1.0" />
+  </settings>
+</component>
\ No newline at end of file
diff --git a/.idea/misc.xml b/.idea/misc.xml
new file mode 100644
index 000000000..f56ad02b9
--- /dev/null
+++ b/.idea/misc.xml
@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" project-jdk-name="Python 3.8 (pythonProject)" project-jdk-type="Python SDK" />
+</project>
\ No newline at end of file
diff --git a/.idea/modules.xml b/.idea/modules.xml
new file mode 100644
index 000000000..69050637e
--- /dev/null
+++ b/.idea/modules.xml
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/y0.iml" filepath="$PROJECT_DIR$/.idea/y0.iml" />
+    </modules>
+  </component>
+</project>
\ No newline at end of file
diff --git a/.idea/vcs.xml b/.idea/vcs.xml
new file mode 100644
index 000000000..94a25f7f4
--- /dev/null
+++ b/.idea/vcs.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>
\ No newline at end of file
diff --git a/.idea/y0.iml b/.idea/y0.iml
new file mode 100644
index 000000000..7bf7bee0d
--- /dev/null
+++ b/.idea/y0.iml
@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="PYTHON_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="jdk" jdkName="Python 3.8 (pythonProject)" jdkType="Python SDK" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+  <component name="PyDocumentationSettings">
+    <option name="format" value="PLAIN" />
+    <option name="myDocStringFormat" value="Plain" />
+  </component>
+</module>
\ No newline at end of file
diff --git a/src/y0/controls.py b/src/y0/controls.py
index 4cb856189..86ceb724f 100644
--- a/src/y0/controls.py
+++ b/src/y0/controls.py
@@ -2,6 +2,7 @@
 
 """Predicates for good, bad, and neutral controls."""
 
+from .algorithm.conditional_independencies import are_d_separated
 from .dsl import Probability, Variable
 from .graph import NxMixedGraph
 
@@ -44,3 +45,27 @@ def is_bad_control(graph: NxMixedGraph, query: Probability, variable: Variable)
     """
     _control_precondition(graph, query, variable)
     raise NotImplementedError
+
+
+def is_outcome_ancestor(
+    graph: NxMixedGraph, cause: Variable, effect: Variable, variable: Variable
+) -> bool:
+    """Check if the variable is an outcome ancestor given a causal query and graph.
+
+    > In Model 8, Z is not a confounder nor does it block any back-door paths. Likewise,
+    controlling for Z does not open any back-door paths from X to Y . Thus, in terms of
+    asymptotic bias, Z is a “neutral control.” Analysis shows, however, that controlling for
+    Z reduces the variation of the outcome variable Y , and helps to improve the precision
+    of the ACE estimate in finite samples (Hahn, 2004; White and Lu, 2011; Henckel et al.,
+    2019; Rotnitzky and Smucler, 2019).
+
+    :param graph: An ADMG
+    :param cause: The intervention in the causal query
+    :param effect: The outcome of the causal query
+    :param variable: The variable to check
+    :return: If the variable is a bad control
+    """
+    if variable == cause:
+        return False
+    judgement = are_d_separated(graph, cause, variable)
+    return judgement.separated and variable in graph.ancestors_inclusive(effect)
diff --git a/tests/test_controls.py b/tests/test_controls.py
index 50e0358cf..0dae7570c 100644
--- a/tests/test_controls.py
+++ b/tests/test_controls.py
@@ -4,7 +4,7 @@
 
 import unittest
 
-from y0.controls import is_bad_control, is_good_control
+from y0.controls import is_bad_control, is_good_control, is_outcome_ancestor
 from y0.dsl import U1, U2, A, M, P, U, W, X, Y, Z
 from y0.graph import NxMixedGraph
 
@@ -95,3 +95,7 @@ def test_bad_controls(self):
         for model in bad_test_models:
             with self.subTest():
                 self.assertTrue(is_bad_control(model, P(Y @ X), Z))
+
+    def test_neutral_controls(self):
+        """Test neutral controls."""
+        self.assertTrue(is_outcome_ancestor(model_8, X, Y, Z))