The name comes from Derek Parfit's famous thought experiment that reveals a fundamental problem for utilitarians. The bar chart above illustrates the paradox: World A has a small population living excellent lives (tall, narrow bar). World Z has an enormous population living lives barely worth living (very long, low bar).

Intuitively, most people think A is better than Z. But Parfit showed we can construct a logical chain from A to Z through seemingly reasonable steps. We start by adding people with good lives to create A, then through gradual changes involving worlds B, C, and so on, we eventually reach Z. Each step seems like an improvement or at least not a worsening.

This is the repugnant conclusion. If we simply add up total wellbeing like hedonistic utilitarianism suggests, World Z could contain more total happiness than World A, even though each individual life in Z is barely worth living while lives in A are excellent.

I find the name fitting because "mere addition" captures exactly what hedonistic utilitarianism does: we simply add up wellbeing across all conscious beings to determine the right action. While the paradox seems troubling, I think we should probably accept it if the premises are sound. In practice, the extreme trade-offs between population size and welfare quality may never actually arise. This blog explores how this framework of adding up wellbeing applies to real moral decisions we face today.