By analyzing the dielectric nonlinearity with the Landau thermodynamic expansion, we find a simple and direct way to assess the importance of the eighth-order term. Following this approach, it is demonstrated that the eighth-order term is essential for the adequate description of the paraelectric-ferroelectric phase transition of BaTiO3. The temperature dependence of the quartic coefficient β is accordingly reconsidered and is strongly evidenced by the change of its sign above 165°C. All these findings attest to the anomalously strong polarization anharmonicity of this material, which is unexpected for classical displacive ferroelectrics.