diff --git a/src/model.rs b/src/model.rs
index 75e1e81..06c959c 100644
--- a/src/model.rs
+++ b/src/model.rs
@@ -66,6 +66,10 @@ impl<B: Backend<FloatElem = f32>> Model<B> {
             * ((-r + 1) * self.w().slice([14..15])).exp()
     }
 
+    fn mean_reversion(&self, init_d: Tensor<B, 1>, new_d: Tensor<B, 1>) -> Tensor<B, 1> {
+        self.w().slice([7..8]) * (init_d - new_d.clone()) + new_d
+    }
+
     fn step(
         &self,
         i: usize,
@@ -82,6 +86,7 @@ impl<B: Backend<FloatElem = f32>> Model<B> {
             let r = self.power_forgetting_curve(delta_t, stability.clone());
             // dbg!(&r);
             let new_d = difficulty - self.w().slice([6..7]) * (rating.clone() - 3);
+            let new_d = self.mean_reversion(self.w().slice([4..5]), new_d);
             let new_d = new_d.clamp(1.0, 10.0);
             // dbg!(&new_d);
             let s_recall = self.stability_after_success(