diff --git a/src/schema/expressionLanguage.test.ts b/src/schema/expressionLanguage.test.ts
index 742300bd..bec4ff7e 100644
--- a/src/schema/expressionLanguage.test.ts
+++ b/src/schema/expressionLanguage.test.ts
@@ -16,16 +16,27 @@ Deno.test('test expression functions', async (t) => {
   })
   await t.step('intersects function', () => {
     const intersects = expressionFunctions.intersects
-    assert(intersects([1, 2, 3], [2, 3, 4]) === true)
+    const equal = expressionFunctions.allequal
+
+    const truthy = (a: any): boolean => !!a
+
+    assert(truthy(intersects([1, 2, 3], [2, 3, 4])))
     assert(intersects([1, 2, 3], [4, 5, 6]) === false)
-    assert(intersects(['abc', 'def'], ['def']) === true)
+    assert(truthy(intersects(['abc', 'def'], ['def'])))
     assert(intersects(['abc', 'def'], ['ghi']) === false)
+    // Just checking values, I'm not concerned about types here
+    // @ts-expect-error
+    assert(equal(intersects([1, 2, 3], [2, 3, 4]), [2, 3]))
+    // @ts-expect-error
+    assert(equal(intersects(['abc', 'def'], ['def']), ['def']))
 
     // Promote scalars to arrays
     // @ts-ignore
-    assert(intersects('abc', ['abc', 'def']) === true)
+    assert(truthy(intersects('abc', ['abc', 'def'])))
     // @ts-ignore
     assert(intersects('abc', ['a', 'b', 'c']) === false)
+    // @ts-expect-error
+    assert(equal(intersects('abc', ['abc', 'def']), ['abc']))
   })
   await t.step('match function', () => {
     const match = expressionFunctions.match
diff --git a/src/schema/expressionLanguage.ts b/src/schema/expressionLanguage.ts
index ee399b35..2dcf029f 100644
--- a/src/schema/expressionLanguage.ts
+++ b/src/schema/expressionLanguage.ts
@@ -40,14 +40,30 @@ export const expressionFunctions = {
     const index = list.indexOf(item)
     return index != -1 ? index : null
   },
-  intersects: <T>(a: T[], b: T[]): boolean => {
+  intersects: <T>(a: T[], b: T[]): T[] | boolean => {
+    // Tolerate single values
     if (!Array.isArray(a)) {
       a = [a]
     }
     if (!Array.isArray(b)) {
       b = [b]
     }
-    return a.some((x) => b.includes(x))
+    // Construct a set from the smaller list
+    if (a.length < b.length) {
+      const tmp = a
+      a = b
+      b = tmp
+    }
+    if (b.length === 0) {
+      return false
+    }
+
+    const bSet = new Set(b)
+    const intersection = a.filter((x) => bSet.has(x))
+    if (intersection.length === 0) {
+      return false
+    }
+    return intersection
   },
   match: (target: string, regex: string): boolean => {
     const re = RegExp(regex)
diff --git a/src/tests/schema-expression-language.test.ts b/src/tests/schema-expression-language.test.ts
index b994fcb2..c6ee58a8 100644
--- a/src/tests/schema-expression-language.test.ts
+++ b/src/tests/schema-expression-language.test.ts
@@ -19,6 +19,7 @@ const equal = <T>(a: T, b: T): boolean => {
 Deno.test('validate schema expression tests', async (t) => {
   const results: string[][] = []
   const header = ['expression', 'desired', 'actual', 'result'].map((x) => colors.magenta(x))
+  const xfails = ['intersects([1], [1, 2])']
   for (const test of schema.meta.expression_tests) {
     await t.step(`${test.expression} evals to ${test.result}`, () => {
       const context = { file: { parent: null }, dataset: { tree: null } } as unknown as BIDSContext
@@ -33,6 +34,13 @@ Deno.test('validate schema expression tests', async (t) => {
           pretty_null(actual_result),
           colors.green('pass'),
         ])
+      } else if (xfails.includes(test.expression)) {
+        results.push([
+          colors.cyan(test.expression),
+          pretty_null(test.result),
+          pretty_null(actual_result),
+          colors.yellow('xfail'),
+        ])
       } else {
         results.push([
           colors.cyan(test.expression),
@@ -41,7 +49,10 @@ Deno.test('validate schema expression tests', async (t) => {
           colors.red('fail'),
         ])
       }
-      assertEquals(actual_result, test.result)
+      // Don't fail on xfail
+      if (!xfails.includes(test.expression)) {
+        assertEquals(actual_result, test.result)
+      }
     })
   }
   results.sort((a, b) => {