To think that fertility -- one of the key issues in evolution -- would not be influenced by genetics requires a massive detachment from reality as well as lack of awareness of current knowledge about genetics and human behavior.
Note that we have data on this, and a good rule of thumb is that most everything is about half genetic. This includes likelihood to have kids. Why? well, things like sperm counts, and personality traits are influenced by genetics, as are things like tendency to be religious[4]. Values are also strongly influenced by genetics, as is income and intelligence[5], all of which is correlated with decisions to have or delay having children. Similarly the duration of the reproductive lifespan of females is influenced by genetics[2] - something very important as women delay childbirth:
The most recent GWAS conducted in ∼370,000 women of European ancestry identified 389 independent signals explaining ∼7.4% of the population variance in age at menarche (Fig. 1), corresponding to ∼25% of the estimated heritability[2]
And even the decision to delay childbirth is influenced by many factors that have a genetic component.
> We don't have a "don't have kids" gene that can get outcompeted.
An international research team, including the Norwegian Institute of Public Health, has found twelve genes that may help determine why some people have children at an early age, while others remain childless.[3]
See also [1] for an overview and discussion.
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[1] https://www.ncbi.nlm.nih.gov/books/NBK97281/
[2] https://www.nature.com/articles/s41525-018-0068-1
[3] https://www.fhi.no/en/news/2016/twelve-genes-influence-ferti...
