Electronic configuration of Thallium is \([ Xe ] \,4 f ^{14} \,5 d ^{10}\, 6 s ^2 \,6 p ^1\).

\(d\) and \(f\) electrons of inner shells provide poor shielding, thus the attraction of the nucleus on the \(s\) electrons of the outermost shell increases. This is called the inert pair effect.

Whereas, \(s\) electrons provide strong shielding to \(p\) electrons, thus making it easy for the \(p\) electrons to be released.

Thus, although Thallium shows \(+3\) and \(+1\) oxidation states, \(+1\) is more common.