[mdiesel]

There are a couple of key differences that I think could lead to a shift towards selfishness.

With the size of populations, there's less feeling of individual impact. If I don't do "my bit" then it's such a miniscule negative to society as a whole, it won't really matter.

We have a relatively new economic principle that if everyone acts in their own best interests, that will also further society's interests. That means there's no moral choice between what benefits me and what benefits others, I can always pick what benefits me.

These aren't universal, but are two simple reasons why selfishness could be more prevalent now than a lot of history.