diff --git a/index.md b/index.md
index 2354fbe..729de1e 100644
--- a/index.md
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@@ -62,5 +62,6 @@ establish virtuous cycles. Let's talk about what we want to see.
 ## Reference
 
 * [What Is Psychological Safety?](https://hbr.org/2023/02/what-is-psychological-safety)
+* [Wikipedia entry: "Psychological safety"](https://en.wikipedia.org/wiki/Psychological_safety)