Monotone homomorphisms on convolution semigroups

We study monotone homomorphisms on the semigroup of probability measures on $\mathbb{R}$, by which we mean maps to the reals that are monotone with respect to the stochastic order and additive under convolution. We show that scalar multiples of the expectation are the unique monotone homomorphisms on the semigroup of measures with finite $p$-th moment, for any $1 \le p < \infty$. We also prove that the entire semigroup of probability measures admits no non-zero monotone homomorphism.