In Group 13 elements, as we move down the group, the tendency of the ns² valence electrons to remain paired and not participate in bonding increases due to the poor shielding effect of the intervening d and f orbitals. This phenomenon is known as the inert pair effect. Consequently, the stability of the lower oxidation state (+1) increases down the group, while the stability of the higher oxidation state (+3) decreases. Thus, the increasing order of stability of the +1 oxidation state is Al < Ga < In < Tl.