Finally, I must admit I've tricked you. I snuck in an assumption while you weren't looking: universality. Remember, that social utility is a weighted sum of individual utilities; however, it's entirely possible for you to believe that these weights should depend on the person. For instance, you might think that a father should weigh his own family members more than that of strangers.
I disagree with this, and for some of you this will be equally obvious. After all, we grow up in a culture strongly influenced by the ideals of the Christianity and the American Revolution.
Love your neighbor as yourself...love him with all your heart, with all your understanding and with all your strength.
- Mark 12:31 - 33, The New International Version
You have heard that it was said 'Love your neighbor and hate your enemy.' But I tell you, love your enemies and pray for those who persecute you... If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your own people, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Father is perfect
- Mark 5:43 - 48, The New International Version
We hold these truths to be self-evident, that all men are created equal...
- Declaration of Independence (US 1776)
There are, however, I think more universal arguments for this.
Suppose you reject universality, then you think that two perfectly ethical people can disagree. More generally, two different countries run by perfectly ethical leaders can disagree.
Imagine, for instance, that the Mexican government valued Mexicans more than Americans and the American government valued Americans more than Mexicans [you don't have to imagine this]. Then, it would be perfectly reasonable for the Mexican government to actively aid Mexicans crossing the border - legalizing and subsidizing the purchase of forged documents, for instance.
And if you're an American, you can't reasonably complain about this, because that's exactly what you'd say America's government should do if the situation was reversed.
Veil of Ignorance
On top of this issue, making ethics non-universal contradicts the intuition behind the veil of ignorance. If you put the people of Mexico and the US together behind a veil of ignorance and make them come up with a set of rules, they'll assign equal weight to everyone to maximize their own expected utility.
You can say that the veil of ignorance should only apply within a society and not between societies. But, this runs into problems:
- The boundaries of societies are inherently grey - not black-and-white.
- What makes national societies the magical level to use the veil of ignorance? Why note the local level? The household level? Alternatively, why not the global level? The universal level?
- The same arguments that apply to using the veil of ignorance within a society act as justifications for it applying between societies.
Friends and Family
As far as I can tell, the main reason people want to be able to assign different weights than other people is so they can value their own friends and family more than strangers.
The first thing to note is that there are other reasons to spend more time thinking and helping friends and family over strangers. After all, you know these people better, making you better able to help them. Moreover, deepening these relationships yields benefits to both of you and makes society a nicer place to live.
All of this is to say, you can operationally care more for your friends and family without making that a fundamental part of your ethical beliefs.
If you're reading this, you're probably in the top 15% of the world by income - 88% of Americans are "title": "How Americans compare with the global middle class". Because the poor need money more than the rich, summatarianism suggests that giving money to the needy is better than keeping it for yourself. If you're in this wealthy group, you have an economic self-interest to oppose egalitarian ethical systems like these.
However, it should be clear that this opposition most likely stems from where you were born. If I knew I was going to be born in the First World, I'd be better off advocating for a non-summatarian ethical system. However, if I didn't know where I was going to be born, I'd very much prefer a summatarian system - and pretty much every rational person would too.
It's worth reiterating: the average person will (by definition) be better off under a summatarian ethical system. Since any rational being would choose it behind a veil of ignorance, if you don't accept summatarianism it follows that the reason is either (1) a rejection of the veil of ignorance or (2) knowledge you've gained about after having been born - that is by biases caused from wining the birth lottery.