Because to the extent that there is a "wanting to have kids" trait directly it was less necessary in the ancestral environment because there was also a "wanting

Because to the extent that there is a "wanting to have kids" trait directly it was less necessary in the ancestral environment because there was also a "wanting to have sex" trait that could do the work. But getting into the stuff we actually measure, stuff like Agreeableness[1] that makes people want to have more kids in the modern world also has significant drawbacks if there's the danger of, e.g., starving to death if you don't look out for number one. [1]https://en.wikipedia.org/wiki/Agreeableness https://en.wikipedia.org/wiki/Agreeableness