It's hard to tell if these studies show that people want fewer children, or if people want fewer children as they age (ie their ideal number declines). It would be also really interesting to see if this is a form of goal post shifting where people who do not have children (for example procrastinate) move the goal post to tell themselves they succeeded at what they _actually_ wanted.

That would take longitudinal studies and to break people into cohorts of how many they wanted at various ages, and how many they infact had. Then see if those who had N children wanted more or less (and was it more or less than their initial predictions, and are they happy with their choices)...

The little bit of googling I've found suggests mostly want as many or more children than they have, meaning having 0 children is a dominated strategy compared to at least having some, if not more than you currently already do -- up to a point.