Given ever-increasing population compared with the fixed size of the Earth, is it ethical for me to want to raise my children in a house with a yard, when a handful of houses could make room for apartments that could house hundreds of people?

I don't see what is wrong with doing things that you want to do in a case like this. One is not perpetually obliged to think of whether one could be doing more for people. Right now instead of responding to this query I might be more suitably employed doling out food for the homeless and the computer on which I am now typing could be sold to provide water for villages in the developing world which require it. I could right now be doing things that save lives, yet here I am selfishly typing away unnecessarily and satisfying my desire to make my opinions public.

To allow all my personal interests to be submerged under the interests of others, though, is to dissolve one's personality. For some of course taking this step is in fact a reflection of their personality, but in the example you say you want to bring your children up in a house with a yard. We are not under the obligation to be saints and there is no reason why you should feel guilty about the reasonable ambition to own a yard.

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