diff --git a/docs/jpsi2pipipi.ipynb b/docs/jpsi2pipipi.ipynb
index 46e9c684..b8aedfbc 100644
--- a/docs/jpsi2pipipi.ipynb
+++ b/docs/jpsi2pipipi.ipynb
@@ -37,6 +37,7 @@
     "from typing import Iterable\n",
     "\n",
     "import ampform\n",
+    "import attrs\n",
     "import graphviz\n",
     "import jax.numpy as jnp\n",
     "import matplotlib.pyplot as plt\n",
@@ -61,7 +62,11 @@
     "from tensorwaves.data.transform import SympyDataTransformer\n",
     "from tensorwaves.interface import DataSample, ParameterValue, ParametrizedFunction\n",
     "\n",
-    "from ampform_dpd import DalitzPlotDecompositionBuilder, simplify_latex_rendering\n",
+    "from ampform_dpd import (\n",
+    "    DalitzPlotDecompositionBuilder,\n",
+    "    set_initial_state_polarization,\n",
+    "    simplify_latex_rendering,\n",
+    ")\n",
     "from ampform_dpd.decay import (\n",
     "    IsobarNode,\n",
     "    Particle,\n",
@@ -149,7 +154,7 @@
    "outputs": [],
    "source": [
     "reaction = qrules.generate_transitions(\n",
-    "    initial_state=INITIAL_STATE.name,\n",
+    "    initial_state=(INITIAL_STATE.name, [-1, +1]),\n",
     "    final_state=[p.name for p in FINAL_STATE],\n",
     "    allowed_intermediate_particles=[\"a(0)(980)\", \"rho(770)\"],\n",
     "    mass_conservation_factor=0,\n",
@@ -282,7 +287,6 @@
   {
    "cell_type": "markdown",
    "metadata": {
-    "jp-MarkdownHeadingCollapsed": true,
     "tags": []
    },
    "source": [
@@ -303,7 +307,16 @@
    "outputs": [],
    "source": [
     "model_builder = DalitzPlotDecompositionBuilder(DECAY, min_ls=True)\n",
+    "initial_polarization = {\n",
+    "    t.initial_states[-1].spin_projection for t in reaction.transitions\n",
+    "}\n",
     "dpd_model = model_builder.formulate(reference_subsystem=1)\n",
+    "dpd_model = attrs.evolve(\n",
+    "    dpd_model,\n",
+    "    intensity=set_initial_state_polarization(\n",
+    "        dpd_model.intensity, initial_polarization\n",
+    "    ),\n",
+    ")\n",
     "dpd_model.intensity"
    ]
   },
diff --git a/src/ampform_dpd/__init__.py b/src/ampform_dpd/__init__.py
index 33b87f33..126bd14d 100644
--- a/src/ampform_dpd/__init__.py
+++ b/src/ampform_dpd/__init__.py
@@ -4,6 +4,7 @@
 import sys
 from functools import lru_cache
 from itertools import product
+from typing import Iterable
 
 import sympy as sp
 from ampform.sympy import PoolSum
@@ -348,6 +349,19 @@ def _print_Indexed_latex(self, printer, *args):
     sp.Indexed._latex = _print_Indexed_latex
 
 
+def set_initial_state_polarization(
+    intensity: PoolSum, spin_projections: Iterable[sp.Rational | float | int]
+) -> PoolSum:
+    """Set the spin projections of the initial state."""
+    helicity_symbol, _ = intensity.indices[0]
+    helicity_values = tuple(sp.Rational(i) for i in spin_projections)
+    new_indices = (
+        (helicity_symbol, helicity_values),
+        *intensity.indices[1:],
+    )
+    return PoolSum(intensity.expression, *new_indices)
+
+
 def _formulate_clebsch_gordan_factors(
     isobar: IsobarNode,
     helicities: dict[Particle, sp.Rational | sp.Symbol],