I think it's a lot more complicated than a simple genetic trait: people weigh the cost and benefits using a lot of no-single-gene high-level reasoning and (in this environment, the massive inequality in haves and have-nots, the relative lack of material support for raising a child from the larger community, the overall ease relative to decades past of access to the tools of controlling pregnancy, and the removal of most of the patriarchal obstacles to women in particular remaining marriage-free) decide that it's not worth the cost. It's not like there's a risk of failure to propagate the species; if anything, the risks seem most associated with increasing the headcount, not decreasing it.
I think it's a lot more complicated than a simple genetic trait: people weigh the cost and benefits using a lot of no-single-gene high-level reasoning and (in this environment, the massive inequality in haves and have-nots, the relative lack of material support for raising a child from the larger community, the overall ease relative to decades past of access to the tools of controlling pregnancy, and the removal of most of the patriarchal obstacles to women in particular remaining marriage-free) decide that it's not worth the cost. It's not like there's a risk of failure to propagate the species; if anything, the risks seem most associated with increasing the headcount, not decreasing it.