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.

Then they are a very poor one since they caused a decrease in the rate of decrease.