It's still dopamine. Dopamine + freedom.

It starts with access to tv, movies, leisure travel, and disposable income.

These things take you out of the evolutionary algorithm. Boredom, uncertainty, low dopamine, mind not preoccupied = having children.

We identified this trend already in DINKs, it just turns out biology's adversarial algorithm is far more widespread than just that one demographic pattern.

Smartphones are an accelerant.