Then I must admit confusion here: when human groups have norms that punish "defectors", genes that predispose someone to play a "tit for tat" strategy (or, to some extent, altruism) rather than defection are rewarded and spread through the gene pool faster. Is that not a case where group-favoring genes become widespread? To the extent it diverges from the definition you gave, that's because of pretty arbitrary caveats.

The definition of group selection, from Wikipedia:

In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.

The key is that the benefit to the group is at least part of what is driving the adaptation. Now an adaptation (like tit-for-tat) can certainly benefit the group, but that doesn't mean there is group selection going on - the benefit to the group has to be part of the cause for the trait's spread, apart from the benefit from the individual.

Tit-for-tat is individually fitness maximizing in many situations. In fact, it's an Evolutionary Stable Strategy. So in a population of tit-for-tat players, it's fitness maximizing to play tit-for-tat. So tit-for-tat is not an example of group selection, or at least it's existence doesn't imply group selection has occurred.

Edit: And what other species has anything like China's one-child policy?

That's a decision of a small group of people imposed on a much larger group of people. If each person was individually choosing to have only one child, then it might be group selection. With that being said, the changing birth patterns of developed countries is an interesting phenomena to consider. It's probably just a case of external conditions changing faster than evolution changes us though.