British pop psychologist Oliver James says Avoid putting the under-threes in daycare if you can.
Here's the study, "Transition to child care: associations with infant-mother attachment, infant negative emotion, and cortisol elevations." James's summary is actually not too bad, at least by Jamesian standards, but it omits a number of important points...
The story starts with cortisol, the hormone we secrete when faced with threat, leading to "fight or flight". Its levels were measured in 70 15-month-old children at home before they had ever been to daycare. Compared with this, the levels had doubled within an hour of the mother leaving them in daycare on the first, fifth and ninth days. Measured again five months later, while no longer double, they were still significantly elevated compared with the home baseline...
There was no control group: all the 70 kids went into daycare. We don't know what would have happened to their cortisol levels if they hadn't; the authors found no evidence that ago alone increases cortisol, but this doesn't mean that staying at home wouldn't have had any effect.
While statistically significant at p=0.01, the rise after 5 months was small, from 0.3 micrograms/dl to 0.4. The earlier rises were larger but entirely expected given that starting daycare, like starting anything new, is temporarily stressful. Temporary stress is part of life.
The 5 month measure was taken in daycare: so all it shows us is that daycare continues to be slightly "stressful", compared to sitting at home; it doesn't show that being put in daycare causes cortisol levels to be raised even when you're not there, which would be slightly more concerning, although still not very.
More fundamentally, James paints cortisol as a bad thing: "high cortisol has been shown many times to be a correlate of all manner of problems, this is bad news." Yet if you don't have enough cortisol, that's called Addison's disease, and you can die of it. Our bodies release cortisol to mobilize us for pretty much any kind of action. Physical exercise, which of course is good for you in pretty much every possible way, cause cortisol release. This is why cortisol spikes every day when you wake up: it helps give you the energy to get out of bed and brush your teeth. Maybe the kids in daycare were just more likely to be doing stuff than before they enrolled. For illustration, here's a stock example of an adult's diurnal (daily) cortisol profile:
Extremely high levels of cortisol over a long period certainly do cause plenty of symptoms including memory and mood problems, probably linked to changes in the hippocampus. And moderately elevated levels are correlated with depression etc, although it's not clear that they cause it. But a rise from 0.3 to 0.4 is much lower than the kind of values we're talking about there. Reading Oliver James's article probably increased my cortisol by at least 0.1 microgram/dl, because I find his writing quite stressful. Maybe I should sue him for hippocampal damages. More seriously though, I bet it increased the cortisol of a lot of mothers, by making them feel guilty about their choice to put their kids in childcare. I have no opinion on whether that's a good idea or not, and I don't think anyone writing a 665 word article for the Guardian can have an informed opinion either. It's your choice, and a tricky one no doubt, as it depends on loads of things like your finances, family situation, your child's personality, whether there are any good daycare facilities nearby, etc. etc. - all of which you're best placed to weigh up. What it doesn't depend on is cortisol. You're capable of judging whether your kid is stressed or not, by looking at them and listening to them, more accurately than someone armed only with a cortisol analysis kit. Neuroscience is no substitute for common sense.