  Given a tree $T$ of order $n,$ one can contract any edge and obtain a new
tree $T^{*}$ of order $n-1.$ In 1983, Jamison made a conjecture that the mean
subtree order, i.e., the average order of all subtrees, decreases at least
$\frac{1}{3}$ in contracting an edge of a tree. In 2023, Luo, Xu, Wagner and
Wang proved the case when the edge to be contracted is a pendant edge. In this
article, we prove that the conjecture is true in general.
