Homomorphisms from automorphism groups of free groups

The automorphism group of a finitely generated free group is the normal closure of a single element of order 2. If $m$ is less than $n$ then a homomorphism $Aut(F_n)\to Aut(F_m)$ can have cardinality at most 2. More generally, this is true of homomorphisms from $\text{Aut}(F_n)$ to any group that does not contain an isomorphic copy of the symmetric group $S_{n+1}$. Strong constraints are also obtained on maps to groups that do not contain a copy of $W_n=(\mathbb{Z}\mathrm{/}2{)}^n⋊S_n$, or of ${\mathbb{Z}}^{n-1}$. These results place constraints on how $\text{Aut}(F_n)$ can act. For example, if $n\ge 3$ then any action of $\text{Aut}(F_n)$ on the circle (by homeomorphisms) factors through \text{\mathrm{det}}:Aut(F_n) \to \Bbb Z_2 .