The effectiveness of generational garbage collection is usually explained through the generational hypothesis, that “most objects die young”.

Despite its simplicity, the generational hypothesis leaves some things to be desired: it is not obvious how it can be measured as a property of a program (independent of a particular GC strategy), it is not composable (in that it does not follow from a larger program by being true of its parts), and even its connection to the effectiveness of generational GC is murkier than it may first appear.

We propose instead lifetime dispersion as a measure of how generational a program’s objects are, and explain how it can be quantified by the Gini coefficient. We show that this measure is both composable, and directly connected to effectiveness of generational collection.