The bits about breastfeeding and the other biological limiting factors (the indirect controls, basically) came to light during Richard Lee's fieldwork with the San and Ju/'hoansi peoples of South Africa in the 1960s.

The bit about active measures is available if you peruse the anthropological literature on the subject (I don't have a specific citation in mind), and the sort of thing covered in introductory classes to the field -- it's common knowledge within that domain.