I remember this little theory where in some ancient cultures children would be weaned around age six and the mothers would only become fertile again after that. I'm not sure how established the mechanicas are, but the theory says that hormonal pathways where breastfeeding causes infertility are activated by scant nutrition. It sounds like an effective way to adjust births to the available resources.
Breastfeeding does reduce odds of conception but it doesn't really last more than a year. Plenty of people get pregnant while breastfeeding.
I dunno, original poster mentioned scant nutrition being a condition for an extended effect so mayyyybe, seems like it would not be very hard to study today.
