The stability of +1 oxidation state among $\text{Al, Ga, In}$ and $\text{Tl}$ increases in the sequence:

The inert-pair effect is the tendency of the two electrons in the outermost atomic-s orbital to remain unshared in compounds of post-transition elements.

Inert-pair effect is seen in the elements of 13th,14th, 15th and 16th groups.

Due to the inert-pair effect elements down the group exhibit oxidation state which is 2 units lesser than the elements above them.

Stability of +1 oxidation state due to inert pair effect is in the order $\mathrm{A}\mathrm{l}<\mathrm{G}\mathrm{a}<\mathrm{I}\mathrm{n}<\mathrm{T}\mathrm{l}$