diff --git a/pkgs/stdenv/generic/make-derivation.nix b/pkgs/stdenv/generic/make-derivation.nix
index 5b446b11826..2bceb9e4333 100644
--- a/pkgs/stdenv/generic/make-derivation.nix
+++ b/pkgs/stdenv/generic/make-derivation.nix
@@ -89,7 +89,10 @@ in rec {
 
     , patches ? []
 
-    , __contentAddressed ? false
+    , __contentAddressed ?
+      (! attrs ? outputHash) # Fixed-output drvs can't be content addressed too
+      && (config.contentAddressedByDefault or false
+          || builtins.getEnv "NIXPKGS_CA_BY_DEFAULT" == "1")
 
     , ... } @ attrs: