The first argument made by the journal (and the Sears pediatricians) is that crying-it-out is stressful for babies, flooding their sweet little brains with hormones such as cortisol that interfere with healthy brain development.
Yes: Chronic, toxic stress is bad for young brains. But several nights of crying during an otherwise happy infancy does not constitute chronic stress. The types of stress that have been shown to cause developmental problems in children include sexual and physical abuse and serious neglect, described as “the absence of sufficient amounts of essential experiences,” which some children raised in state-run institutions in China and Romania suffered, for instance.
The Sears family also cites a study showing that babies who cry for prolonged periods have lower IQs than kids who don't, but the study looked at kids who happened to cry a lot more than usual, not those who were left alone to cry.
(The Sears family confused the direction of the causal arrow in the study, too, because the researchers concluded that “underlying neurological problems may be the cause of prolonged crying,” not the other way around.)
Crying's effect on cortisol
When Time magazine senior editor Jeffrey Kluger contacted the authors of some of the studies cited by William Sears for an article he wrote in May 2012, the scientists said it was unfair that Sears had used their work as evidence against sleep training.
“Our paper is not referring to routine, brief stressful experiences, but to abuse and neglect,” explained Yale psychologist Joan Kaufman. “It is a miscitation of our work to support a non-scientifically justified idea.”
There is, however, one study cited multiple times in Clinical Lactation that specifically assesses the effects of crying-it-out on infant cortisol levels. In this 2012 study, University of North Texas educational psychologist Wendy Middlemiss and her colleagues tracked the behavior and cortisol levels of 25 infants, ages 4 to 10 months, as they went through a five-day sleep training program a la crying-it-out in a sleep lab in New Zealand.
The researchers measured the blood cortisol levels of the infants and their mothers before and after the babies were put to sleep on the first and the third nights. On the first night, the babies cried, but their cortisol levels did not go up upon being left to cry themselves to sleep. On the third night, none of the babies cried much at all, and their cortisol levels stayed constant. In other words, the sleep training worked.
What Middlemiss considers a cause for concern is that the babies' cortisol levels never dropped during the course of the study (the mothers' cortisol levels did, on the third night). “What's dangerous about the situation,” Middlemiss explained to me, “is that the mother has no idea – because the behavioral indication, i.e., the crying, is absent – that the infant had remained stressed.”
How do we know the infants were stressed to begin with if their cortisol levels never rose? Middlemiss says that the babies in her study must have been stressed as soon as they arrived at the sleep lab – it's a foreign environment – so it didn't matter that the training itself didn't incite a further increase.