Cumulative, accumulative, additive, summative are comparable when meaning increasing or produced by the addition of like or assimilable things.
Something is cumulative which is constantly increasing or is capable of constant increase (as in size, amount, power, or severity) by successive additions, successive accretions, or successive repetitions; thus, the cumulative effect of a drug may be harmful even though the immediate effect of each dose has, apparently, been beneficial; terror is cumulative because one fear tends to inspire another.
Something is accumulative which is constantly increasing in amount or bulk through successive additions or which has reached its sum total or magnitude through many such additions.
Cumulative is now used more often than accumulative especially where increasing severity or enhancement in influence or power are to be suggested.
Something is additive which is of such a nature that it is capable either of assimilation to or incorporation in something else or of growth by additions.
An additive detail, element, or factor is one that has such affinity for another thing that it becomes a constituent part of that thing; thus, red, green, and blue-violet are the additive colors and are used in color photography because they blend to form any color.
Something is summative which is capable of association or combination with other things so as to produce such a sum total as an additive whole or a cumulative effect.