Why can mercury form both +1 and +2 oxidation states?

Mercury is unique among the zinc group elements because it can form both +1 and +2 oxidation states.1

While zinc and cadmium typically lose two electrons to form dipositive ions ($M^{2+}$), mercury's ability to exist in the +1 state is due to the formation of the diatomic mercurous ion, $[H{g}_2{]}^{2+}$. In this ion, two mercury atoms are linked by a covalent bond, with the two electrons shared between them.1

In contrast, mercury in the +2 oxidation state forms the $H{g}^{2+}$ ion. While compounds of mercury in the +2 state are generally covalent, mercury(I) compounds are typically ionic. This ability to form stable metal-metal bonds is a distinct characteristic of mercury that differentiates it from the other elements in its group.1