Some of it is definitely cultural, but the section of the doc that mentions these economic shocks (across time) segues into the pattern of unexpected / unwanted childlessness. These would be populations where the cohort of 0 children (as opposed to just fewer children) surged. I gather the point it was making was that where and when the childless statistics surged it was largely due to economic shocks.

The 0 children cohort is important because in many places it surged rapidly, it brought down the rate to below replacement levels. It also signals a failure to "get off the ground" as opposed to having 1 kid and stopping by choice.

EDIT: It's the section of the 'vitality curve'