Let's put that aside
While I would believe that "girl" fetuses have higher survival rate than "boy" fetuses when the mother is not healthy, I do not get how that would skewer the demographic in the other direction in the other direction when the mother is healthy.
The main argument against this point is the one I researched later on. Which is that at conception things are very close to 50:50, and then things skew
either male or female. Here's some more research:
https://www.ncbi.nlm.nih.gov/pubmed/3071849Parents in higher socioeconomic classes are more likely to have sons, but the effect is largely due to the excess male mortality during most of the gestational period.
This is from 1988, and they also cite the difference as merely being the higher male mortality in the womb. However ... more recent data collected on conception rates doesn't show any excess of original male embryos that would be needed for this factor to be relevant. So it does seem to really skew either one way or the other, not just one way.
The thing is, we can't ever say for sure whether this phenomena is adaptive or just coincidence. However, does it make a whole lot of difference? e.g. we can argue about
why the sky is blue, but the various arguments don't nullify the statement "the sky is blue".
If more boys are born in good times, to wealthy families, to strong mothers etc etc ... that has
real social and demographic implications, and those implications exist irrespective of why the phenomena exists.