I'd love to see the underling data, and get some input from a statistician/demographer. (maybe we can have a dedicated thread for dissecting statistics and studies?)
It would seem to my untrained statistical head, that given exponential growth of a community the decline in average family size should be true, absent all other factors.
The number of chareidi women 30 and under HAS TO be a much greater percentage of the total childbirthing cohort (women 18+) than it was 20 years ago.
(As always, the underwriters of the study do have a bias...)
This is true, the data needs more context. It would be a lot more insightful if the data showed average births by (women's) age and seeing how it evolves over time for a 25/30/35/40 year old. We would be able to measure, among other things, couples choosing to have fewer kids or choosing to have more children at a later age.