What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?

Within the main group elements, the ionization enthalpy decreases regularly as we move down the group due to the following two factors:
(i) Atomic size: As we move down the group, the atomic size of the atom regularly increases due to the addition of one new principal energy shell at each succeeding element.
(ii) Screening effect: With the addition of new shells, the number of inner electron shells, which shield the valence electron increases. In other words, shielding effect or screening effect increases.
As a result of both these factors, the force of attraction of the nucleus for the valence electrons decreases. Thus, the electron becomes less and less firmly held to the nucleus as we move down the group. Hence there is a regular decrease in the value of ionization enthalpy on moving down the group.