One of the fundamental challenges of parenting is that the practice is also the performance; childcare is all about learning on the job. The baby is born, a lump of need, and we’re expected to keep her warm, nourished and free of diaper rash. (Happy, too.) A few random instincts kick in, but mostly we just muddle our way through, stumbling from nap to nap, meal to meal. Or at least that’s how it feels to me.

Given the steep learning curve of parenting, it’s not surprising that many of us yearn for the reassurance of science. I want my sleep training to have an empirical basis; I’m a sucker for fatty acids and probiotics and the latest overhyped ingredient; my bookshelf groans with tomes on the emotionally intelligent toddler.

Occasionally, I find a little clarity in the research. The science has taught me about about the power of emotional control and the importance of secure attachments. But mostly I find that the studies complicate and unsettle, leaving me questioning choices that, only a generation or two ago, were barely even a consideration. I’m searching for answers. I end up with anxiety.

Consider the kindergartner. Once upon a time, a child started kindergarten whenever they were old enough to make the age cutoff, which was usually after they turned five. (Different states had slightly different requirements.) However, over the last decade roughly 20 percent of children have been held back from formal schooling until the age of six, a process known as “redshirting.” The numbers are even higher for children in “socioeconomically advantaged families.”

What’s behind the redshirting trend? There are many causes, but one of the main factors has been research suggesting that delaying a child’s entry into a competitive process offers lasting benefits. Most famously, researchers have demonstrated that Canadian hockey players and European soccer stars are far more likely to have birthdays at the beginning of the year. The explanation is straightforward: because these redshirted children are slightly older than their peers, they get more playing time and better coaching. Over time, this creates a feedback loop of success.

However, the data has been much more muddled when it comes to the classroom. Athletes might benefit from a later start date, but the case for kindergartners isn’t nearly as clear. One study of Norwegian students concluded that the academic benefits were a statistical illusion: older children score slightly higher on various tests because they’re older, not because they entered kindergarten at a later date. Other studies have found associations between delayed kindergarten and educational attainment – starting school later makes us stay in school longer – but no correlation with lifetime earnings. To make matters even more complicated, starting school late seems to have adverse consequences for boys from poorer households, who are more likely to drop out of high-school once they reach the legal age of school exit.

Are you confused? Me too, and I’ve got a got a kid on the cusp of kindergarten. To help settle this debate, Thomas Dee of Stanford University and Hans Henrik Sievertsen at the Danish National Centre for Social Research decided to study Danish schoolchildren. This is for two reasons: 1) the country had high quality longitudinal data on the mental health of its students and 2) children in Denmark are supposed to begin formal schooling in the calendar year in which they turn six. This rule allowed Dee and Sievertsen to compare children born at the start of January with children born just a few days before in December. Although these kids are essentially the same age, they ended up in in different grades, making them an ideal population to study the impact of a delayed start to school.

After comparing these two groups of students, Dee and Sieversten found a surprisingly large difference in their mental health. According to the scientists, children who were older when they started kindergarten – they fell on the January side of the calendar – displayed significant improvements in mental health, both at the age of 7 and 11. In particular, the late starters showed much lower levels of inattention and hyperactivity, with a one-year delay leading to 73 percent decrease in reported problems.

As the scientists note, these results jive with a large body of research in developmental psychology suggesting that children benefit from an extended period of play and unstructured learning. When a child is busy pretending – when they turn a banana into a phone or a rock into a spaceship – they are practicing crucial mental skills. They are learning how to lose themselves in an activity and sustain their own interest. They are discovering the power of emotion and the tricks of emotional control. “To become mature,” Nietzsche once said, “is to recover that sense of seriousness which one had as a child at play.” But it takes time to develop that seriousness; the imagination cannot be rushed.

Does this mean I should hold back my daughter? Is it always better to start kindergarten at a later date? Probably not. Dee and Sieverten are careful to note that the benefits of a later start to school were distributed unevenly among the Danish children. As a result, the scientists emphasize the importance of taking the individual child into account when making decisions about when to start school. Where is he on the developmental spectrum? Has she had a chance to develop her play skills? What is the alternative to kindergarten? As Dee noted in The Guardian, “the benefits of delays are unlikely to exist for children in preschools that lack the resources to provide well-trained staff and a developmentally rich environment.”

And so we’re left with the usual uncertainty. The data is compelling in aggregate – more years of play leads to better attention skills – but every child is an n of 1, a potential exception to the rule. (Parents are also forced to juggle more mundane concerns, like money; not every family can afford the luxury of redshirting.) The public policy implications are equally complicated. Starting kindergarten at the age of six might reduce attention problems, but only if we can replace the academic year with high-quality alternatives. (And that’s really hard to do.)

The takeaway, then, is that there really isn’t one. We keep looking to science for easy answers to the dilemmas of parenting, but mostly what we learn is that such answers don’t exist. Childcare is a humbling art. Practice, performance, repeat.

Dee, Thomas and Hans Henrik Sievertsen. "The Gift of Time? School Starting Age and Mental Health,” NBER Working Paper No. 21610, October 2015.