This is going to sound like massive overkill, but it is well known that the only 1-dimensional polynomial representations of $GL(V)$ (which is what you're looking at) are the nonegative powers of $\mathrm{det}$.
Reference (I assume from the mention of statistics that you are OK working with base field $\mathbf{R}$ or $\mathbf{C}$): e.g. Procesi's Lie Groups on p.278 lists all rational representations as all $$S_\lambda(V)\otimes\mathrm{det}^k,\qquad k\in\mathbf{Z},$$ and on p.270 gives a dimension formula for $S_\lambda(V)$ which is $>1$ unless $S_\lambda(V)$ is trivial.