Or assumes their grandparents are allowed within 200 feet of any children.

Fairly normal if having kids before 20 is normal.

And I suspect disaster to have correlated chances of killing parents or grandparents or children. i.e. if you've become a parent there's a good chance your parents did too.

Our own lived experiences are likely outliers, since developed countries have different outcomes than the past.

> if you've become a parent there's a good chance your parents did too

Crap and nonsense! Rewrite:

Once someone's survived to become a parent, there's a good chance the grandparents survived as well (assuming the norm is to breed ASAP).