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 metals.
[1] The name suggests that the outermost s electron pairs are more tightly bound to the nucleus in these atoms, and therefore more difficult to ionize or share.
As a result, the inert pair of ns electrons remains more tightly held by the nucleus and hence participates less in bond formation.
[4] The higher value for thallium compared to indium is partly attributable to the influence of the lanthanide contraction and the ensuing poor shielding from the nuclear charge by the intervening filled 4d and 5f subshells.
[2] That said, the authors note that several factors are at play, including relativistic effects in the case of gold, and that "a quantitative rationalisation of all the data has not been achieved".
[9] Another example are some thallium(I) salts where the asymmetry has been ascribed to s electrons on Tl interacting with antibonding orbitals.