Biology isn't my strong suit, so I hope that I'm not misunderstanding you, but wouldn't your statement imply selection at the group level?
Additionally, although the figures I quoted were over a two-year figure, they exemplify a larger trend. For example, the Wikipedia article on Australia's demographics show that Australia hasn't had replacement level fertility since 1975.
My statement really should have been worded as a question as biology, admittedly, it is not my strong suit either so I won't attempt to address your question but hope that someone with a better understanding will chime in here.
Additionally, although the figures I quoted were over a two-year figure, they exemplify a larger trend. For example, the Wikipedia article on Australia's demographics show that Australia hasn't had replacement level fertility since 1975.