The repugnant conclusion is a notorious problem in the field of population ethics. It was created by philosopher Derek Parfit and presented in his book Reasons and Persons. He states it as follows:
For any perfectly equal population with very high positive welfare, there is a population with very low positive welfare which is better, other things being equal.
Let's walk through the explanation to see how he arrives at this conclusion.
To understand this problem, we'll have to have a metric for measuring welfare. Of course, it's impossible to simply quantify welfare, but let's imagine we could compress everything good or bad about life into one number. We'll presume that a score of 100 is the best possible life (think Heaven) and a score of -100 is the worst possible life (think Hell). Negative scores are so bad that the people living them would rather die. An individual with a score of 0 has no preference between living or dying.
Imagine starting with a small population, say of 10 people living on a planet. These people have incredibly high welfare. In fact, they are living the best possible lives—a welfare of 100. Let's call the universe they live in "Universe A".
That's great for those people. This universe is a very good place. Now, let's imagine we can select between two universes - A and B. Universe B has the exact same planet with the same people living the same perfect lives, but in addition, there is a second planet many galaxies away. The planet contains twice as many people who are also living excellent lives, though not perfect ones. On this planet, there are 20 people living with a welfare that is 90% as good as those on the other planet. The people from the two planets will never know of each other or interact in any way. The question is, which universe do you prefer: A or B?
Most people will choose Universe B. The argument is that Universe B adds more happy people to the universe, so why wouldn't it be better? We can also plot the total happiness to see how much it has increased.
Now, let's consider Universe C. In Universe C, the two populations are combined and the welfare is 95 - lower for the 10 at 100 but higher for the 20 at 90. So there's an overall increase in the weighted average welfare. How do you feel about Universe B versus Universe C? Note that in comparing the two universes, we're not talking about "taking" welfare from the 10 and redistributing it to the 20. We're talking about making both universes from scratch.
Most people would say that they prefer Universe C because it has a higher average welfare and is more equal.
So far, nothing truly repugnant has happened, although you might start to see how it could come about. We can continue this trend, comparing Universe C with D just like we did with A and B, and then D with E, just like we did with B and C. Let’s take a look at Universe E.
And if we look at the total welfare, we'll see that it keeps increasing.
If we continue this trend of selecting better and better universes, eventually we will end up with a universe with zillions of people with a welfare just above 0. That is, zillions of people whose lives are barely worth living. Repugnant indeed.
But I would argue that this isn't a repugnant conclusion. To see why, let's take a look at some of the intermediate universes to get an idea of what the extreme ones would look like. Let's look at Universe Z.
Universe Z consists of over 15 million people with an average welfare of over 50. This is a lot of people living very good lives. I would argue that this universe is much, much better than a universe of only 10 people living perfect lives.
Let's go further. Let's take a look at Universe 100 (I had to switch to numbers, for obvious reasons).
These numbers are overwhelming; the scale on the welfare graph is in units of 10^25. I don't know what these numbers are, but it's so, so, many people, and so, so much welfare. If every person on Earth suddenly became their own planet Earth each containing the total population of Earth, it wouldn't be nearly this many people. That's how many people were talking about.
But, we have to admit, a welfare of 8 is a far cry from the welfares of 90 and 100 that we started with. However, these people have lives that are worth living. They still enjoy life, overall. They still prefer living to not living. We have traded unimaginable blissful welfare for 10 for a life worth living for so many more. This is a trade worth taking.
Let's look at the far extreme to see the Repugnant Conclusion. As we continue making trades, improving our possible universe bit by bit, we’ve driven the average welfare down near 0.
The number of people has increased to astronomical proportions. Numbers of this magnitude are truly mind-boggling. Trying to comprehend them is a futile task, but let’s try just for fun. Imagine meticulously counting each star in the Milky Way and, once done, passing a baton over to the next person to do the same. This continues, person by person, until every individual on Earth had their turn. And for each completed round of counting by all of humanity, you'd take a single grain of sand from our vast beaches and mark it red. If you repeated this process until every grain of sand on the planet was red, the total number of stars counted... still wouldn't be close. You'd need to do this process every day for millions of years before you came close.
And don't forget, the minimum welfare is not 0; it's -100. The people in these universes still prefer living to not living, albeit marginally. I think the reason that this seems repugnant is that our brains don't work with numbers this large. And because we cannot comprehend it, we discount its significance.
So in the extreme, we have created a world where an unimaginable number of people would say, "I am glad to be alive." This, surely, is a wonderful thing.