Not yet, all rumours re’ the effects of breastfeeding aside. They sprang back just fine and dandy, thanks. 
And the nipples return to their original sensitivity, I was very pleased to discover 
This isn’t true in the United States or in any industrialized society and it most certainly isn’t true worldwide.
Even if it were true, how can you possibly say that wet-nursing, which has been around for thousands of years, has been too recent to be an explanation, but something that has happened in one generation is a explanation?
The easiest answer is that not being able to breastfeed a baby doesn’t affect reproduction. Obviously the mother can reproduce, because she has. And there is no reason for the child to die, because wet-nursing or formula can replace breastmilk. Wet-nursing is hardly something confined to the upper classes or to recent eras. All healthy tribal societies have a multitude of nursing mothers who can and do take on another baby in case of emergency. That is wet-nursing just under a different cultural identity. There is not the slightest reason to assume that this would affect genes at any speed.
If breastfeeding or the lack of it has no effect at all on reproduction, then those genes are never removed from the gene pool. Nothing more is needed. Basic genetics.
There has been a trend against reproduction in recent history IMHO. Infertility issues and lack of breast milk. As for why this is, I will offer some possibilities, one is that modern medicine has made it possible for couples who normally could not have children to conceive, passing on this problem to their children. Another is the society pressure for formula.
Another is the use of formula and breast milk from a bottle (obtained by a pump), instead of direct from breast to baby has weakened the mother-baby bond, the mother’s body in response does not produce the quality and quantity of milk it should, I also believe other aspects of modern society, such as mother working, having others raising their baby, weakens this bond as well. And there is the cycle of simply believing you can not produce enough milk has the very effect.
Women have historically typically worked while they had babies. Unless they were so rich that they didn’t ever need to work.
In the past there were no doubt a certain number of women who couldn’t breastfeed. If they couldn’t find another woman or two to take over, or didn’t have a handy lactating mammal like a goat or cow, the baby died. Just one more factor in infant mortality rates.
Yes, modern society interferes with nursing, too, but that doesn’t mean everything was all fine and dandy way back when. A lot of babies used to die, just as a lot of women used to die in childbirth.
But jobs today are much less conducive to having a woman breast feed her baby while she works. A woman working in a field, carrying the baby with her constantly is going to be able to breast feed easier than a woman who must be separated from her child for 10 hours or more a day.
Just thought of another point -
Women today typically have children at a much older age then women thousands of years ago did. Is it easier for young moms to produce milk than older ones? I would assume so, if it’s easier for them to get pregnant, but I don’t know.
As already mentioned, women’s work was usually conducive to interruption and child-tending and breastfeeding. That’s a large part of why women historically got work that could be interrupted and that was usually safer for small children–spinning, weaving, cooking, fieldwork, poultry or dairy work, etc. (Did you know that cloth production usually took more time and effort than food production?) A woman with small children, who was nursing each child for a couple of years, could work all day and feed the baby at the same time, unlike today. And much of it was cooperative–think of, say, ancient Egyptian women employed to weave in a large room–the women would have the small children with them, so there was a lot of supervision and cooperative work. Or extended family living in a compound/home, so that a grandmother could tend children while mom went on a short errand.
All that is nothing like our industrialized work today, so it’s not much of a comparison. Modern working women have to work against the system to get any success with nursing a child; there’s almost no accommodation for a simple biological fact, and what there is is grudgingly given.
The modern-day concept of hiring a “wet-nurse” is too recent to make the hypothesis that humanity has evolved to need a wet-nurse to ensure sufficient milk production. (Shared nursing responsibilities among mothers in small tribal groups is a different story)
Relevant or not, no evolution is required for the explanation that modern women are just having children at a later age than they used to.
To continue my comments on working women, it’s also worth remembering that until pretty recently, a woman’s main job in life was to bear and raise children. It was a simple necessity. For example, on average, an ancient Greek married couple needed to bear 6 children just to keep the population stable*; therefore a woman’s primary job was to raise the next generation and keep her society going. The clear alternative was conquest or starvation, which was all too likely regardless. All other work came second to that, so naturally a woman’s other work would have to be adapted to the necessities of pregnancy and child-rearing. It’s a much more obviously crucial job when you really are thinking about survival all the time.
Nowadays we live in a completely different sort of world. There’s just no equating a modern working mom with her ancient sisters and saying “Well, women have always worked” as if the work was at all comparable.
*Or so said my classics professor, and my own reading seems to bear that out.
My aunt successfully breastfed her first four children, so presumably she both knew what she was doing and made enough milk. After the fifth was found to be essentially starving to death, they put the baby on formula. Kids six through nine were successfully breastfed. Before formula, without a wet nurse or other outside help, number five would have died (she’s now a happy, healthy teenager), but the other eight would have made it, or at least not died from lack of breastmilk. The inability to nurse that one child wouldn’t have taken my aunt’s genes out of the pool, but my cousin would have been dead all the same.
Lots of good answers to what is undoubtedly a multifaceted phenomenon. I’d add another: “good nutrition” is only a good thing so far. Obese women have more trouble breastfeeding than healthy weight women. One reason for this may be that excess estrogen (stored in bodyfat) seems to counteract some of the prolactin hormones that trigger milk production. We probably have more obese women that at most points in history, and can therefore reasonably expect a greater rate of hypolactation.
T’other thing I’d like to add is that we expect a phenomenal growth rate out of our infants, after 80 years of charting them on cow’s milk and formula. When a human milk fed infant doesn’t grow at the same rate as a cow milk fed baby, the doctor will often express concern and counsel supplemental feedings. We’re using formula as the baseline for “normal”; instead of being concerned that perhaps formula fed babies are growing too fast, we worry that breastfed babies are growing too slowly, and so we feed them like cattle to speed things up.
And finally, of course, it bears repeating: we don’t even know if this is really a phenomenon. We don’t have good numbers for how many women *needed *wet nurses, goats or gruel in the past, versus who *chose *to use them, and we don’t even have those numbers for today. Infant mortality being what it was and is, there may be no answer because there’s no change. Every species produces more babies than will survive to adulthood; hypolactation may be one of the reasons why.
Sure, but it certainly could have, if your cousin was your aunt’s first child, and your aunt died giving birth to her second.
I think people are reading negativity into the OP because of the use of judgmental-sounding language:
“A substantial number of women … say they cannot produce”
“the claim is simply that their bodies will not produce milk.”
It sounds like the poster doesn’t believe these women. That’s probably why some are taking issue with the question.
True, but a big difference is that women took their babies with them to work and/or worked from the home and so could breastfeed at will. Not so in the modern system where work and home and family are disconnected to the point we need child care and formula.
I was fortunate enough to be able to take my baby to work (small business owner) and not have to worry about pumping in some stinking restroom. I worked 50 hrs a week starting from when she was a week old; wore her in a sling, had a place for her in my office, tended to her as needed, was wonderful.
Traditionally, women have always taken their babies to work with them or worked from the home or, as a last resort, had relatives care for them during work hours. It is not “either/or”; it IS possible to work AND parent/breastfeed.
I think you’re right, but it’s actually just simple factual reporting. Some women say they cannot produce enough milk; they claim that their bodies will not produce milk. Can they? We don’t know. All we know is that they self-report that they cannot, using unknown means, produce “enough” milk, whatever that means.
Certainly SOME of them haven’t tried, or haven’t tried very hard. Some of them do say that they couldn’t make enough, when what they meant was they couldn’t do it in a time frame or manner comfortable for them, and so they chose formula or supplementation instead. Some of them say it to get breastfeeding advocates off their backs, when they really never intended to breastfeed at all and never tried it. And yes, some of them really did try every technique and medication in the book and their children didn’t thrive on their milk alone, but thrived on supplementation or formula feeding. It’s only that last group that the OP sounds interested in, but it’s impossible to tell how large that group is when all three use the same language to describe their experience.
First step: define terms and gather data. We don’t really know what we’re dealing with until we can better define what “substantial” “cannot” “produce” and “enough” all mean in reality.
What she said. OTOH, there are some women who (like the woman cited above who failed with one child in the middle of her child-bearing, when she successfully nursed all the others) honestly try very hard, and fail.
In general, women who fail at nursing do so either because they have conflicting obligations (usually a job), or because they aren’t trying very hard. In lots of places here in the US, people look at a nursing mother strangely, no matter how modest and discreet she may be about it when it becomes necessary to feed the baby in public - and sometimes even when they’re visiting in someone else’s private home. And most new mothers aren’t exhibitionists, so that’s a deterrent.
I had only one child, but had no difficulty whatever at nursing him. However, I did have a job, so he had to have a bottle when I was at work. By six months he didn’t seem to care which he had. Because the received wisdom at the time was that those first six months were the crucial period, I switched him over to formula, for the sake of convenience. I had to work.
It’s not possible for an outside observer to determine which is the case for any given mother and baby. I, too, wonder to what degree kids today are growing too fast and too soon. And that very possibly is the result of formula feeding in infancy, bolstered by the far poorer nutrition of today thanks to fast food. Fifty years ago, most kids got fast food as a very occasional treat (maybe once a month, if that often). We know that fast food is high in calories - mostly from fat and sugar. And the meat is permeated with growth hormone and antibiotics from the animals being fed that to expedite production. Veggies? Don’t make me laugh. But it can’t be established without some sort of scientific study. I hope somebody’s doing it.
When I was in HS, I was one of the three tallest girls in my class, at 5’8" (one other was the same, the third was an inch taller). It is not in the least unusual now to see girls of that age who are 6’ tall - or taller. The difference can’t be because any of my classmates were undernourished. We lived in the far suburbs of Detroit, and at least a third of the class were the offspring of professionals or small business owners. More than half the class had parents who worked in the auto industry. The UAW contracts of that time had already made them financially able to give their kids all they needed - food, clothing, and college, if the kids so desired. I don’t recall anybody in my school who looked malnourished (quite the contrary), or who ever gave the impression that they didn’t get all the food they wanted. But it was, by and large, a far healthier diet.
Is anyone else thinking of the line from Dune, “They tried and failed?” “No, they tried and died.” 
Valete,
Vox Imperatoris
For me, this is reason #1. We’ve changed from a cooperative society to an individual society where “breastfeeding assistance” means you get a handful of visits from a lactation consultant while in the hospital, and you get sent home with a breastfeeding pamphlet.
#2, which goes hand in hand with #1… breastfeeding is work. Newborns do not eat, they get fed. The woman has to do 90% of the work, the baby has an instinctual ability to cry when hungry and suck when something is in its mouth, and that’s about it. Lots of times, they don’t even suck right, the woman has to make sure he’s latched on correctly. Saying “breastfeeding is natural” is like saying “hunting is natural” or “gathering is natural”. Sure, we’ve done it forever, but you can’t throw a modern man out into the wilderness with nothing more than a pamphlet and expect him to survive. Yet, we expect women to breastfeed “naturally”.
Taken together, we’re asking women to do work that may often require assistance or support from experts. We take away the readily available experts that existed for millenia, and replace it with snippets of information. We make it less “acceptable” to openly breastfeed. We also offer a reasonably suitable alternative (formula) that requires less expertise, and is more acceptable in polite society. It’s no surprise that many women wind up having problems with breastfeeding.
There is one common nursing difficulty that certainly doesn’t explain all nursing difficulties but happens often enough to mention here.
Frequently the “couldn’t produce enough milk” phenomenon can be attributed to formula suplementation. A frequent slippery slope is:
Mother nurses. Baby eats constantly because that’s what new babies do.
Mother/doctor/well meaning relative expresses concern that baby is eating “too often” or not on the desired spot on the (formula based) growth charts (as mentioned above by WhyNot).
(for the purpose of this example I am excluding situations wherein the nursing relationship is difficult from the beginning for various reasons)
Mother suplements with formula “just to top baby off” or to get a break from round the clock nursing.
Milk production is a supply and demand system - the baby demands more milk by nursing more, the body amps up production. If that extra demand is being met by formula, then the body will not increase production.
Result: milk production insufficient for baby’s growing needs.
If a wet nurse or jar of Similac wasn’t around, the mother would either nurse more and produce more milk, or the baby would fail to thrive assuming more milk couldn’t be produced.
If round the clock breastfeeding were socially acceptable and more widely understood as normal (at least in infants) there would be less pressure to supplement.