Ice Water, Math Quizzes, and Shocks: The 1948 Breastfeeding Experiment That Showed Stress Can Slow Milk Flow

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ALEX MITA | AFP via Getty Images Share on Twitter Share on Facebook Share on Pocket Long before lactation consultants talked about stress and milk supply, a nursing mother in the 1940s agreed to an experiment that sounds almost cruel today. While she fed her baby, researchers dipped her feet in ice water, pulled on her big toe until it hurt, and fired difficult math problems at her, delivering a shock for each wrong answer.

Each time, the baby got less milk. The study, published in 1948 by Dr. Michael Newton and Niles Newton, became an early experimental look at how distress can interfere with the human milk letdown reflex, and its lesson still shapes how clinicians think about breastfeeding support nearly 80 years later.

The experiment had an unusual ancestor. In work published in 1941, dairy scientists Fordyce Ely and William Petersen frightened cows during milking by placing a cat on a cow’s back and exploding paper bags every 10 seconds. The cows stopped releasing milk. That study earned a posthumous 2024 Ig Nobel Prize in Biology.

Ely and Petersen also found that injecting adrenaline stopped milk ejection much as fright did, while injections of posterior pituitary extracts, sold as Pitocin and Pitressin, started the milk flowing again within about 30 seconds. The open question was whether the same switch worked in people.

The Newtons set out to test it. Michael Newton was a physician, and Niles Newton would go on to become one of the best-known breastfeeding researchers of the 20th century. Their paper, “The let-down reflex in human lactation,” appeared in The Journal of Pediatrics in 1948.

The method was simple. The researchers weighed the baby to measure how much milk it took during feedings when the mother was relaxed and when she was stressed.

The stressors were deliberately unpleasant. Obstetrician Alison Stuebe, writing in a 2009 essay on the experiment, described how the researchers dipped the mother’s feet in ice water, asked her complicated math problems and shocked her when she got them wrong, and pulled on her big toe until it hurt.

The result was consistent: under stress, the baby got less milk. The authors concluded that mothers who are stressed or in pain can have trouble letting down milk, and that their frustrated, crying babies can make the mothers more stressed, setting up a vicious cycle.

The letdown reflex is driven by oxytocin, a hormone released when a baby suckles that squeezes milk out of the glands and into the ducts. The National Library of Medicine’s MedlinePlus notes that pain, stress, and anxiety can interfere with that reflex. Niles Newton later published further work on the quantitative effect of oxytocin, sold as Pitocin, on human milk yield.

If stress blocks oxytocin, the obvious next idea is to give oxytocin to struggling mothers. It has not panned out as a simple solution.

An early trial of oxytocin nasal spray in mothers of premature infants reported a large effect, but it was stopped after only eight first-time mothers had been studied, according to the National Library of Medicine’s LactMed database.

A later randomized, double-blind trial of 51 mothers expressing milk for babies born before 35 weeks found that total milk production did not differ between women using oxytocin spray and those using a placebo. Most mothers believed they were getting the real drug, which the researchers said suggested a significant placebo effect, along with possible benefits from the extra support mothers received during the study.

Stuebe, who later served as president of the Academy of Breastfeeding Medicine, drew a broader lesson from the Newton experiment, arguing that the answer is to reduce the stress new mothers face. “Effectively, the American new mother has her feet firmly planted in ice water,” she wrote in 2009.

The 1948 study involved a single mother and her baby, so it cannot show how common or how large the effect is across women. Later research has broadly supported the idea, with important caveats.

A review in Clinical Therapeutics, published online in late 2021, found evidence linking maternal psychological distress to delayed onset of full milk production and shorter exclusive breastfeeding. But the authors noted that results were inconsistent, study methods varied widely, and the biological mechanisms remain unclear. They also described the relationship as two-way, since breastfeeding problems can themselves cause distress.

One small study the review described found that noise or mental arithmetic, stressors echoing Newton’s experiment, reduced and delayed oxytocin pulses during suckling in 22 women five days after giving birth.

Newton kept pursuing the mind-milk connection. In a study reported by TIME in 1950, she asked 91 expectant mothers at the Hospital of the University of Pennsylvania whether they planned to breastfeed. Of those who said they were going to or would love to, 74% did, compared with 26% of those who preferred the bottle.

Nursing parents who notice slow letdown, pain, or concerns about supply should talk with a pediatrician, obstetric clinician, or certified lactation consultant. They can check latch, feeding patterns, and medical causes before anyone assumes stress is to blame.

What did the 1948 Newton experiment test?

Whether stressing a nursing mother during feedings reduces how much milk her baby gets.

Ice water on the feet, painful pulling on the big toe, and hard math problems with a shock for wrong answers.

Each time the mother was stressed, the baby got less milk.

Does oxytocin nasal spray help nursing mothers today?

A randomized trial of 51 mothers of premature infants found no difference in total milk production compared with placebo.

Research suggests stress can interfere with oxytocin release, but findings are mixed. Anyone with letdown or supply concerns should consult a clinician or lactation consultant.