A heuristic for the distribution of point counts for random curves over a finite field

How many rational points are there on a random algebraic curve of large genus $g$ over a given finite field ${\mathbb{F}}_q$? We propose a heuristic for this question motivated by a (now proven) conjecture of Mumford on the cohomology of moduli spaces of curves; this heuristic suggests a Poisson distribution with mean $q+1+1\mathrm{/}(q-1)$. We prove a weaker version of this statement in which $g$ and $q$ tend to infinity, with $q$ much larger than $g$.