This is the phenomenon which arise due to the inability of inner electrons to take part in the chemical reactions.
For example in case of group 15 we have Phosphorus, Arsenic , Tin and Bismuth. They all show the oxidation states of +3 and +5, but as we move down the group we will see that the +3 oxidation state becomes more stable. This is due to the inability of inner electrons to take part in reaction.
These are more closely held by the nucleus.