With regards to the public conversation aspect: in particular, what was the nature of advertising around, say, baby formula in the U.S. around that time? And to what degree did the medical field participate in deciding when was the "right time" for weaning? And how, if it all, did those two things intersect?
My source for the numbers is Trends and Differentials in Breast Feeding: An Update by Hirschman and Butler, in the Feb 1981 edition of Demography. The authors speculate on various factors in that article and also repeat various interpretations that may not be their own, and they offer extensive demographic data, but also note that there are probably multiple possible factors that were difficult to pin down at the time.
Thanks!
I can't really speak to the public conversation other than in the broadest possible terms, but a lot of it is tied up in two factors: an increase in food safety, and a decrease in cost of evaporated milk, which was the main ingredient in infant formula.
Before the advent of modern formula, babies who were not breastfed by their mothers would be fed by whatever means possible (and, yes, babies did die sometimes, and frequently suffered nutritional deficiencies). Wet nursing was ideal, if a family could afford it. Watered-down animal milk would be used, or "formulas" made of soaked and mashed grains.
As industrialization spread, things happened to the food supply and long story short, there was a period in America where it was very hard to know if what you were getting was safe to consume, including milk. Conditions from cow to bottle were deeply unsanitary in many cases. Milk would be watered down (to increase profit) and dyed with plaster dust (to make it look whiter). Liquified calf brains would be added to give the appearance of hearty cream. Poor refrigeration and hygiene in on dairy farms and in transit resulted in milk that was full of bacteria and spoiled rapidly, so farmers would add formaldehyde, which needless to say is not something good to ingest, particularly not for babies. All in all, breastfeeding was generally the cheapest, most accessible, and safest option.
When pasteurization came around, it was obviously a godsend. Even the most honest dairy farms had problems with bacterial contamination, and pasteurization practically eliminated those problems while also significantly increasing the shelf life of milk. By the 1930s, pasteurization had become much more common, and by 1947 it was required (fun fact: requiring pasteurization was first proposed by a bacteriologist called Alice Evans in 1918, but her findings were largely ignored because she was a woman and didn't have a doctorate).
Additionally, unsweetened evaporated milk was first created in 1883, and became steadily more affordable over the next decades. By the 1940s, many American babies were fed evaporated milk mixed with water and sugar. In the 1950s, specialized formulas were created, brands which are still around today such as Enfamil and Similac. Formula feeding was convenient -- no need for mom to always be on feeding duty and no need to pump, which went nicely in hand with the first-wave feminist movement. Formula was created with science, which in the 40s and 50s, was generally marketed (with success) as being better.
So it was, in many ways, a combination of increased safety and accessibility of non-human milk products combined with marketing and changing social roles. Milk went from a total crapshoot (maybe it's good and wholesome, or maybe it's got calf brains and formaldehyde in it) to something reliably safe to something specifically designed by Science! to feed your baby.
Some sources: The Poison Squad, by Deborah Blum A Concise History of Infant Formula (Twists and Turns Included) A History of Infant Feeding