K vs. r Selection in Cultivars’ Birth Rates
In evolutionary terms, enduring cultures are neither K nor r-selected (K-selected species are characterized by having a relatively small number of offspring in which they invest high amounts of parental care whereas their r-selected counterparts typically have many offspring but invest less energy in producing them). Enduring cultures have a lot of kids and put more effort, collectively, into child rearing.
Humans in general may have a uniquely high capacity to both have many kids and invest a lot in their upbringing given their (unusual) ability to leverage their own offspring as tools (i.e., people to help the family business, help out around the house, till the fields, provide additional income streams, etc.). When kids aren’t a pure resource drain, it makes sense to not only have more kids but invest more into raising them—all to ensure they yield a high return on investment.
Enduring cultures are not purely r-selected (focused on having tons of kids but investing little in their upbringing) because r-selected cultures are more likely to become diluted through conversion to other cultures in just a few generations. Think about it: If your parents largely ignored you, would you feel a strong devotion to their culture?