  Let $\Sigma$ be a closed surface of genus least two and $\rho \colon
\pi_1(\Sigma) \to G$ a Hitchin representation into
$G=\text{PSL}(n,\mathbb{R})$, $\text{PSp}(2n,\mathbb{R})$, $\text{PSO}(n,n+1)$
or $\text{G}_2$. We consider the energy functional $E$ on the Teichm\"uller
space of $\Sigma$ which assigns to each point in $\mathcal{T}(\Sigma)$ the
energy of the associated $\rho$-equivariant harmonic map. The main result of
this paper is that $E$ is strictly plurisubharmonic. As a corollary we obtain
an upper bound of $3 \cdot \text{genus}(\Sigma) -3$ on the index of any
critical point of the energy functional.
