Unlike `"In"^+, Tl^+` is most stable with respect to disproportionation reaction.

Explanation

Both In and Tl show+ I and + 3 O.S. But inert pair effect increases downwards i.e. more in Tl than In i.e + 1 O.S. of Tl is more stable than +3 O.S. But +3 O.S. of In is more stable than + I O.S. In aqueous solution `In^(+)` undergoes disproportion reaction to form more stable `ln^(3+)` but in Tl, + 1 O.S. is stable. So it does not undergoes disproportion reaction to form + 3 O.S. of Tl.
`3 In^(+) (aq) overset(Disproportion)rarr 2In(s) + In^(3+)` (aq),
`3 Tl^(+)(aq) 2Tl(s) + Tl^(3+)(aq)`.