Cubic interaction vertices in higher spin theories

Based purely on symmetry considerations, we derive the following result: in momentum space, the coefficient of the cubic interaction vertex for a spin $\lambda$ field is equal to the corresponding Yang-Mills (spin 1) coefficient, raised to the power $\lambda$. This result is valid for all $\lambda$ for Lagrangians that contain a cubic interaction vertex of the form $\lambda$-$\lambda$-$\lambda$, in four-dimensional flat spacetime. For $\lambda=3$, we present an additional derivation of this result.