diff --git a/filestore.go b/filestore.go
index 3a497f0..1809268 100644
--- a/filestore.go
+++ b/filestore.go
@@ -92,19 +92,17 @@ func readEntriesFromReader(r io.Reader) ([]PathEntry, error) {
 
 var mu sync.Mutex
 
-// writeFileStore ages the entries if they have grown past the rank threshold,
-// then writes them out.
-func writeFileStore(entries []PathEntry) {
-	var cumulativeRank float64
-	for _, entry := range entries {
-		cumulativeRank += entry.Rank
-	}
-
+// writeFileStore ages the entries if the store has grown past the rank
+// threshold, then writes them out. priorRank is the total read out of the data
+// file: fasd sums the ranks of the lines it reads, before applying any
+// rank + 1/rank increment and without counting the paths being added, so the
+// caller supplies that rather than the post-update total.
+func writeFileStore(entries []PathEntry, priorRank float64) {
 	// Apply decay if cumulative rank exceeds threshold
 	const threshold = 2000.0
 	const decayFactor = 0.9
 
-	if cumulativeRank > threshold {
+	if priorRank > threshold {
 		logger.Log.Println("Rank threshold met. Decaying...")
 		for i := range entries {
 			entries[i].Rank *= decayFactor
@@ -217,6 +215,14 @@ func AddToStore(paths ...string) {
 
 	entries = pruneDecayed(entries)
 
+	// Totalled here, after the prune and before any increment, because that is
+	// what fasd's `count += $2` accumulates: the ranks of the surviving lines
+	// as they were read, with the paths being added excluded.
+	var priorRank float64
+	for _, entry := range entries {
+		priorRank += entry.Rank
+	}
+
 	now := time.Now().Unix()
 	seen := make(map[string]bool, len(paths))
 
@@ -254,5 +260,5 @@ func AddToStore(paths ...string) {
 	}
 
 	// Write updated entries back to the file
-	writeFileStore(entries)
+	writeFileStore(entries, priorRank)
 }
diff --git a/main_test.go b/main_test.go
index 8cb7295..b0a3ebe 100644
--- a/main_test.go
+++ b/main_test.go
@@ -815,3 +815,83 @@ func TestAddToStoreCountsRepeatedPathsOnce(t *testing.T) {
 		t.Fatalf("Expected a single entry at rank 1, but got %v", entries)
 	}
 }
+
+func TestDecayIgnoresNewlyAddedPaths(t *testing.T) {
+	cwd, readEntries := setupProcTest(t)
+
+	high := filepath.Join(cwd, "high")
+	first := filepath.Join(cwd, "first")
+	second := filepath.Join(cwd, "second")
+	for _, dir := range []string{high, first, second} {
+		if err := os.MkdirAll(dir, 0700); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// Just under the threshold. Counting the two new paths would push the
+	// total to 2001 and age the store, but fasd's `count += $2` only sums the
+	// lines it read, so 1999 stands and nothing is aged.
+	writeEntries([]PathEntry{{Path: high, Rank: 1999, LastAccessed: 100}})
+
+	AddPaths([]string{first, second})
+
+	expected := map[string]float64{high: 1999, first: 1, second: 1}
+	assertRanks(t, readEntries(), expected)
+}
+
+func TestDecayIgnoresTheRankIncrement(t *testing.T) {
+	cwd, readEntries := setupProcTest(t)
+
+	high := filepath.Join(cwd, "high")
+	if err := os.MkdirAll(high, 0700); err != nil {
+		t.Fatal(err)
+	}
+	// Exactly at the threshold, which is not over it. Re-adding this path
+	// raises it to 2000.0005, but fasd totals the pre-increment rank, so the
+	// store is not aged.
+	writeEntries([]PathEntry{{Path: high, Rank: 2000, LastAccessed: 100}})
+
+	AddPaths([]string{high})
+
+	assertRanks(t, readEntries(), map[string]float64{high: 2000 + 1.0/2000})
+}
+
+func TestDecayIgnoresPrunedEntries(t *testing.T) {
+	cwd, readEntries := setupProcTest(t)
+
+	high := filepath.Join(cwd, "high")
+	faded := filepath.Join(cwd, "faded")
+	fresh := filepath.Join(cwd, "fresh")
+	for _, dir := range []string{high, faded, fresh} {
+		if err := os.MkdirAll(dir, 0700); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// fasd accumulates count inside the `$2 >= 1` block, so a line below the
+	// prune threshold contributes nothing to the decay decision.
+	writeEntries([]PathEntry{
+		{Path: high, Rank: 1999.5, LastAccessed: 100},
+		{Path: faded, Rank: 0.9, LastAccessed: 200},
+	})
+
+	AddPaths([]string{fresh})
+
+	expected := map[string]float64{high: 1999.5, fresh: 1}
+	assertRanks(t, readEntries(), expected)
+}
+
+func assertRanks(t *testing.T, entries []PathEntry, expected map[string]float64) {
+	t.Helper()
+
+	if len(entries) != len(expected) {
+		t.Fatalf("Expected %d entries, but got %v", len(expected), entries)
+	}
+	for _, entry := range entries {
+		want, ok := expected[entry.Path]
+		if !ok {
+			t.Fatalf("Unexpected entry %v", entry)
+		}
+		if math.Abs(entry.Rank-want) > 1e-6 {
+			t.Fatalf("Expected %s at rank %v, but got %v", entry.Path, want, entry.Rank)
+		}
+	}
+}