> Is it the existence of a rule forbidding it? In that case the principle seems more to be about the rule than about "what if everyone did it?".

The vast majority of the US lives outside what we’d consider “the city” so that’s a bad example. But anyway the difference is that the act in question involves payment for a scarce resource. If everyone paid for a house in the suburbs (assuming that the prices factored in externalities) then it would be fine because prices would regulate land use. And if the price doesn’t include externalities then it is antisocial to live in the suburbs, as folks here in HN point out regularly.

To me it feels like you are arguing that the guiding principle for whether an action is "antisocial" is whether you are free riding or imposing externalities: getting a benefit at other people's expense.

Which is a fine moral principle, and does easily distinguish between all your "antisocial" examples and my "housing choice" example (assuming correct prices).

But it doesn't seem to require the "what if everyone did it?" principle. So I'm wondering if that principle brings any additional value, or indeed if it is a real moral principle at all. Hence my question.

If everybody lived in the suburbs there would be endless rows of cookie-cutter houses with a law in the front and a swimming pool in the back. You would have loops upon loops of cul-de-sacs that just meandered in residential housing blocks. Every 50 miles or so you would have a mall with box stores and you grocery stores would be so big that they would have little electric cars in them to get from one side to the other. Because shopping would be such a big PITA, people would buy mayonaise in 1 gallon jars and would buy vans just so they could transport all of their groceries for a month. Children would have to be bussed to school because the low population density would make a school within walking distance too expensive. Likewise children would be shipped around in their parent's van because soccer fields and basebase fields would be too distant to get to by foot. Public transport would be so expensive that you couldn't have a reliable and prompt service for any reasonable price. People would drive one or two hours to get to work -- each way. Since they would be either commuting or working for 11-12 hours a day, a couple would need 2 vehicles: the van for buying monthly supplies and carting the kids around as well as another vehicle to go to work. The increased load on the traffic system would mean that it would be almost crazy to consider riding a bicycle. All the roads would be set up for cars because it would be the only reasonable transportation that could work. The expensive of maintaining 2 cars, driving all over the city all the time, keeping the house in shape and the garden in shape would mean that in many households both parents would have to work to make ends meet. The children would have to be looked after by a service for most of the day, every day of the week.

Of course many people can't afford to buy a house in the suburbs. Thank goodness.

You basically just described the suburbs of an Australian city. The only difference is we have way more supermarkets than that.

Man, I wish I could have quickly thought of a different example. I tried to avoid starting a housing debate with "assume for the sake of argument," but it doesn't feel like it worked!