The Inert Pair Effect

[Frederick Keith_4C]

So to my understanding, the inert pair effect is when an element forms a a compound with a valency two lower than the expected group valency. In group 13, Al forms Al 3+ ions but In can form In 3+ and In + ions. I don't really understand why this happens? I think it has to something to do with the s-block electrons or maybe the p-electrons but I'm not really sure exactly.
So the question is what causes the inert pair effect???

[Drew Myers 4G]

The reason for the effect is the inertness of the inner s electrons due to poor shielding.

[Julianna Laurentano 4G]

Basically the inert pair effect is when electrons in the outer s orbital remain unionized, and this is because there is poor shielding of these orbitals, so they cannot form bonds. As a result, these electrons remain unionized.