For the social sciences, there is a natural reality .

For the social sciences, the conditions for a "natural experiment" are really great, and one way to create / discover a "natural experiment" is policy change. The same group of people can find many interesting things when they catch up with the policy and fail to catch up with the policy.

Recently, researchers at the University of Barcelona and the University of Pompeii Fabra in Spain published a study after Spain introduced the policy of "parental leave with parents" in 2007. That year, Spain stipulated that fathers could take two weeks of maternity leave, which was extended to five weeks last year.

Researchers found that fathers who took maternity leave were more involved in parenting activities, while mothers were more likely to return to work after childbirth. In fact, this has largely achieved the original purpose of the policy.

However, to the surprise of researchers, fathers who have taken maternity leave or are about to take maternity leave want fewer children to some extent, 7-15% fewer than fathers who have not taken maternity leave. Mothers’willingness to have children increased. This is probably because father’s involvement in the process of child-rearing will lead to a more realistic estimate of the cost of child-rearing and put the quality of child-rearing ahead of quantity.
Of course, there may be other reasons for the decline in fertility willingness (such as the pressure of the economic crisis), but at the same time, the rise in women’s fertility willingness is actually a better signal.