Childcare : what the science says

This article sprouted out of a rather rash promise. I recently posted about massive scientific errors in a self-professed ‘data driven parenting guide’. This was the very first comment:

Great, so if you were to write a science based, research backed rebuttal to this chapter alone, it would recommend what exactly? No daycare? Daycare in particular family instances at particular ages?

I ask knowing full well I likely won’t be able to meet the ideal, but I personally would prefer to know what that ideal is to attempt to get closer to it.

While I know the research here, I’ve always been reluctant to write about it: everything’s so politicised and parents (understandably) get very upset if they don’t think they’re doing the best by their children. So I promised to DM the commenter. And then this happened…

I can see this getting out of hand. Either I write something public or I’ll be getting DMs about this for years…

Dear reader, please stop reading

So. Look. People get very upset when they feel that science disagrees with their personal choices, whether that’s on climate change, vaccination, GMOs or childcare. You probably have strong views on childcare. If so, please stop reading. You’re not going to get anything out of this, and I hate making people unhappy.

Still here? Be warned: this is what I tell friends who haven’t made childcare decisions yet. It gets pretty downbeat at times, because the science is pretty downbeat. If your children are in school, I wouldn’t tell you about the downsides of daycare, because it’s not going to do you any good. If you don’t have choices about childcare, it’s certainly not going to do you any good. Please stop reading and don’t make yourself miserable for no reason.

Science and Authority

Books on parenting usually do this…

That’s the exact opposite of science. The whole point of the Enlightenment, of the dawn of reason, was that we moved away from appeals to authority.

The only reason to believe a scientific finding is that someone points you at the supporting research. After each section below, I’ve linked to the most important research on the topic.

For academics: I’m discussing topics where the evidence from psychology, biology and economics all converge, so that we can be quite confident about findings. If you feel anything I assert is not adequately justified, please let me know here — but please make sure to properly absorb the evidence from outside your own discipline before you do. (Also bear in mind that I am writing for a general audience! Look at the substance, not the language.)

Center Care

There’s a lot more research on center-based daycare than on other childcare options. I’ll go over that first and then talk more briefly about in-home care, etc.. Effects are always measured relative to children staying at home.

One important caveat: most research looks at the average effect on children. Many children are affected much more or less than average. I’m not going to write ‘on average’ before every sentence — but, please, take it as read.

Summary of effects

First, here are the effects of 15–30 hrs of daycare a week, broken down by age.

  • For ages 3+, there are few downsides and substantial advantages. Daycare boosts both cognitive skills (literacy and mathematics) and social skills as measured in the first few years at school.
  • For age 2, the findings are more mixed. This is the best age to start in terms of boosting later cognitive skills, but children are more likely to act out and be angry when they reach school.
  • For age 1, childcare may improve cognitive skills a little, though certainly less than starting at age 2. But it also has even larger negative effects on later behavior in school. There is no boost to social skills.
  • For children aged 0–12 months, daycare likely damages cognitive skills and children’s later behavior at school is even worse. There is no boost to social skills.

If there’s one thing you take away from this post, please let it be this: age makes a huge difference.

Compared to 15–30 hours, 30+ hours a week before 4 or so doesn’t give any cognitive benefits but makes children more likely to act out and be angry when they reach school. For children from “middle class and affluent families”, that much time in daycare has about two-thirds the negative effect on behavior of having “a moderately depressed mother”. Children spending long hours in any kind of out-of-home childcare have been found to be three times as likely to have “elevated levels of aggression”.

Note the age and long-hours effects are separate: putting 6-month-old children in daycare for long hours has a particularly large negative effect on later behavior.

All of that is for an average income family. Low-income children benefit from starting earlier, and high-income children from starting later. (It’s likely not just actual income that matters, but socioeconomic factors — but income is easy to measure objectively so researchers measure it.) The most deprived children can actually benefit from starting as 1-year-olds.

Age matters. Hours matter. Income matters.

Sources: The effects on cognition and the link between center care and later externalising behavior (“acting out”, etc.) at schoool have been found many times. See e.g. (NICHD, 2002), (NICHD, 2004),(Melhuish, 2004) and (Stein, 2013). (Loeb, 2007) is the best single source, though, as it splits results by age and income, and considers ages 0–5. Its findings about young children are borne out by later studies such as (Fort, 2016), (Kottelenberg, 2014) and (Morrissey, 2010). Over a dozen papers have shown negative effects of non-parental care in the first 12 months; see (Im, 2018) for a review. For differences in outcome by family income, see (Melhuish, 2015) and also (Votruba-Drzal, 2004) for cognitive aspects.

(McCartney, 2010) and (Huston, 2015) investigate the causes of the behavioral effect. If you’re looking into that yourself, watch out for studies that rely on maternal assessments of behavior; those were found to be highly unreliable and were actually abandoned by the NICHD research team in the mid 2000s. See (Berg-Nielsen, 2012) and also (Solheim, 2013) for a discussion. Reviews tend to be contaminated by studies with maternal questionnaires, so you really need to look into the original papers here.

Cognition vs Behavior

Those results might suggest a trade-off between cognitive skills and behavior, but that’s a shaky interpretation for two reasons:

  • Parents likely impact cognitive skills more than daycare, but daycare impacts behavior somewhat more than parents. So e.g. parents can improve children’s literacy by reading to them at home, but it’s much harder to fix behavioral problems caused by daycare.
  • Cognitive boosts probably fade out, although it’s not completely settled; skills needed to regulate behavior persist through life.

Sources: See e.g. (NICHD, 2002) study on parenting, but watch out for the fact that their measures include the effect of shared genes. For cognitive skills, contrast e.g. (Baker, 2019) and (Magnusun, 2004) which find fade-out with (Vandell, 2016) which shows persistence. On behavior, (Vandell, 2020) traces more time in daycare centers through impulsivity, risk-taking, behavior problems and police contact up to age 26, and has the relevant references to the older work.

(Baker, 2019) is a particularly useful large real-world study of the universal childcare expansion in Quebec; it gives some of the strongest evidence we have that cognition fades out while behavioural effects persist, and indeed that all the phenomena discussed in this article happen ‘in the wild’. I’ll say more about it below.

Social Skills

The part that I most often see parents get wrong is this: they assume sending children under 2 to daycare will improve their social skills. It doesn’t.

Children don’t normally start playing alongside each other until 30 months or so; playing together starts around 36 months. For children under 30 months, their social environment isn’t other children; it’s you, their primary carers, who teach them about turn-taking and so on. Other infants are too chaotic and inconsistent to learn from. (Ever seen a baby try to crawl over another baby?)

From the first textbook on play I had at hand.

In daycare, each baby or infant will have less attention from caregivers (just because of adult-to-child ratios), so their social skills will develop more slowly. In addition, the stress of the setting, discussed next, likely makes it harder to learn generally.

Sources: this is very standard material, so easier to digest from textbooks than papers. Look at this or any other textbook on play. The original research goes back to 1929.

Cortisol and Stress

Why does daycare usage affect some children’s behavior years down the line? There’s strong evidence that it relates to children’s stress levels in daycare.

Measuring stress observationally is known to be unreliable, so researchers normally look at levels of cortisol, a stress hormone. There are many studies showing that e.g. substance abuse during pregnancy or growing up in adversity permanently affect children’s cortisol response, and that in turn has long-term consequences for behavior, memory and likelihood of mental illness.

The normal pattern (blue line below) is for cortisol levels to start high in the morning and decline during the day.

From a meta-analysis of 7 studies of cortisol in daycare. Note the broken y-axis — afternoon cortisol levels in daycare are about 40% higher than at home, not 100% higher.

In daycare (pink line above) cortisol levels go up and up through the day. That is, children become increasingly stressed throughout whole-day daycare. This finding is very consistent and turns up in studies in (at least) the US, France, Germany, the Netherlands and Norway.

The graph above is for children in daycare morning and afternoon: children who are in for half-days show a less elevated cortisol response or even a normal one. This fits with the finding that children who spend 30+ hours a week are more likely to show behavioral issues later on in school. Higher quality settings cause smaller cortisol rises, which fits with their having smaller effects on behavior (as discussed below). Elevated cortisol depresses the immune system, which is one reason children catch so many bugs in childcare.

An aside: the research tends to focus on the long-term effects of stress, but I find that a little too clinical. If it turns out that we are making toddlers so stressed that we can measure effects on their behavior five years later, I find that upsetting in and of itself. Plus long-term risk of mental illness worries me more than bad behavior.

It’s not clear why long hours in daycare make children so stressed. It was originally thought that the general chaos, noise and exposure to large numbers of infants might cause it. That’s not it: cortisol rises even in settings where a single ‘childminder’ takes care of 3–4 children in their own home (in-home daycare).

Sources: the pathway from early childhood stress to an altered cortisol response to later health issues and mental illness is well established; (Hunter, 2011) is a review and (Miller, 2016) presents relevant references compactly. For the childcare-cortisol link specifically, the classic review/meta-analyses are (Geoffroy, 2006) and (Vermeer, 2006). The review (Vermeer, 2017) updates the findings. Following references from those Vermeer articles reaches most other sources, so I won’t repeat them here. But note especially (Roisman, 2009), which finds time in daycare centers predicts altered cortisol levels at age 15. (Drugli, 2017) is too new to be mentioned in the reviews.

Quality

High quality daycare reduces all the negative effects I’ve discussed above, although it doesn’t eliminate them. It also has long-lasting positive effects on educational outcomes. That’s the good news. The bad news is that it’s hard to find; only 10–15% of daycare in the US is high quality, and many child care centers offer “disturbingly low quality programs”.

My friends always say that they’ve personally been lucky and found fantastic daycare settings. The sad thing is that researchers have found that nearly all parents rate their settings as great. Compared to trained observers, parents overrate infant-toddler rooms by about two standard deviations. That’s like saying parents overrate heights by six inches or IQs by 30 points!

The literature on child care quality is extensive. I’m not going to try to summarise it here. I know some of you will want to learn to evaluate quality yourself, but it’s not something you can pick up from an internet article — or at least, I worry that I’ll do more harm than good if I try.

Well, maybe one simple exception: look for settings with as few children per adult as possible. That’s been shown to make a difference.

Sources: Many sources in ‘Summary of effects’ above also discuss the effect of quality. (Cryer, 1997) is the classic on parents overrating quality. If you want to look into quality more, start with (Burchinal, 2017), which reviews the ‘modest’ links between care quality and outcomes; its bibliography points you at the key studies.

Effect Sizes

Researchers talk about small, medium and large effect sizes. Those are technical terms. Even Cohen, who invented them, warned against using them out of context. Unfortunately, they’re abused all the time to push various agendas to laypeople. I want to try to give you a sense of how large daycare effect sizes are in a way you can understand.

Suppose we were talking about an (imaginary) nutritional program. Rather than telling you it has a small-to-medium effect size, it’s more helpful to say that it will, on average, make your children an inch taller as adults. You can get a sense of what that means. It won’t make them into giants, but it’s noticeable. An extra inch would push them from the 50th percentile into the 63rd percentile. In a class of 30 lined up by height, they’d move up about 4 places.

That’s the rough average effect size for all the things that affect children. Breastfeeding, lead exposure, maternal depression all seem to come in at around “one inch” of whatever each of them affects — IQ, behavior or whatever. Some are a little larger, some a little smaller, but they’re all around an inch. For middle-to-high income children, the effect of long daycare hours on behavior also falls into this range.

There are a few important things to note:

  • Even with the boost from that nutritional program, many children will grow up to be short. (Just, on average, an inch less short than they would have been!) Similarly, even with extensive exposure to daycare, many children will grow up without behavioral problems or later mental health issues. Daycare and the other factors shift the odds; they don’t guarantee bad outcomes.
  • A small shift in the average makes a big difference to the tail. If everyone was an inch taller, there’d be three times as many American men over 6'4". The same applies here: the ‘inch’ of worse behavior caused by long hours in childcare results in triple the number of children with ‘elevated aggression’.
  • Remember that average effects are measured. Some children will be much more affected and some less or not at all. Also remember research shows parents are bad at seeing the behavioral effects of daycare on their own children.

Sources: this is mostly basic statistics, but also draws on numbers from the sources mentioned in ‘Summary of Effects’ above. There’s also a lot of research on what factors make children more or less affected by daycare; see ‘Further Reading’ below for more on that if interested.

Other forms of childcare

To run a good scientific study of two alternatives, you need a couple of hundred data points in each bracket. It’s (reasonably) feasible to find and engage 200 children in daycare and 200 cared for by their mothers. It’s much harder to round up, say, 200 children cared for by nannies. As a result much of the research compares daycare to maternal care; research on anything else is sparser, so this will be a short section. Here are the key points:

  • All forms of relative care seem to be as good as each other. Mothers, fathers, grandmothers — doesn’t matter. Indeed, there are large studies comparing single fathers and single mothers and the differences are minimal.

Let me emphasise here: I have never seen any research supporting the idea that women are better than men at caring for children.

  • Time with professional childminders (a.k.a. in-home daycare providers) can cause later behavioral problems, but much less so than daycare centers. Childminders do not however boost cognitive skills of older children as half-days in daycare do. (Usual caveat: such boosts probably fade out, whereas behavioral effects have long-term consequences.)
  • How do nannies (who come into your home) compare to professional childminders (in their own homes)? Research is sparse, but it seems that nannies may cause fewer emotional problems than childminders, and (like childminders) do not give cognitive boosts.

Sources:

Many of the sources I discuss in the ‘Summary of Effects’ section also talk about relative care and non-center out-of-home care, especially (Morrissey, 2010), (NICHD, 2004) and (Stein, 2013).

Beyond those, (Downey, 1998) is a (large) comparison of single fathers and single mothers. For grandmother care, see (Fergusson, 2008). Note that both these last papers fail to correct for extensive multiple hypothesis testing, and so statistically speaking we would expect them to find one or two positive hypotheses among the mass they test even if all null hypotheses hold. Both do, and I have not read too much into it as AFAIK their findings have not been replicated. In both papers the results for externalising behavior in particular are negative.

If you read into this yourself, you may find assertions that non-maternal childcare is worse than maternal childcare. This is only true in the sense that if you group together children cared for by fathers, grandmothers, childminders and daycare centers, the average effect is worse than maternal care. That effect is driven by the non-relative care, not the fathers/grandmothers/other relatives.

Turning to nannies vs childminders, (Stein, 2013) compares them directly and finds that time with childminders causes more emotional and behavioral problems than time with relatives or nannies. (It actually finds that childminders are on par with daycare, but that is certainly not consistent with other research.) (Dettling, 2000) and (Groeneveld, 2010) found that cortisol levels rose through the day with low-quality childminders; (Gunnar, 2010) finds that process quality affects cortisol but that structural quality does not. (Structural quality is very objective things like caregiver training and experience; process quality is fuzzier things like whether caregivers are warm and caring.)

Further Reading

If you want to look into this more, there are a number of general reviews. (Bradley, 2007) and (Philips, 2010) are a bit outdated but short and readable. (Burchinal, 2015) has the more recent work; one thing to watch out for is that although it highlights issues with maternal questionnaires, it doesn’t catch all the papers that rely on them. (Melhuish, 2015) is also good, though be sure you look at the ‘general population’ half rather than the ‘disadvantaged backgrounds’ half. (Jacob, 2009) is a widely cited review of the effects of childcare on behavior. (Sylva, 2010) is a whole book of findings from a large British study.

I’ve left out some topics to keep this article at a sane length. Quality, for reasons mentioned above. Attachment theory, which goes a long way to explaining why younger children find daycare so stressful. How child temperament interacts with everything else. And finally gender — many studies suggest that behavioural effects are worse for boys, but I worry that people are simply picking up the fact that boys are more likely to act out to start with.* The reviews talk about all of these.

*This has been shown to be true by studies funded by your tax dollars!

Universal Childcare

In the first version of this article, I wrote a little about the evidence from economics on universal childcare programs. I was trying hard to keep it short, but that caused misunderstandings. So in more depth:

Quebec

Quebec implemented universal childcare from birth in 1996. The US NBER summarised the key paper in an article called ‘Canada’s Universal Childcare Hurt Children and Families’. Some quotes:

Along virtually every one of these dimensions [of child well-being], children in Quebec see their outcomes deteriorate relative to children in the rest of the nation over this time period.

[..] this policy resulted in a rise of anxiety of children exposed to this new program of between 60 percent and 150 percent, and a decline in motor/social skills of between 8 percent and 20 percent.

[..] families became more strained with the introduction of the program, as manifested in more hostile, less consistent parenting, worse adult mental health, and lower relationship satisfaction for mothers.

  • The paper picks up something that is specifically happening to preschool children, ramping up from 1996 to 2001. Nothing happens to older children, which makes it unlikely that it’s e.g. the Quebec economy.
  • Quality standards were increased in 1996 as part of the program roll-out. Overall quality was actually pretty bad, but still ‘comparable to the quality of care provided in many other countries’. 25% of the centers were good quality, which is about double the figure for the US.
  • Economists Steven Lehrer and Mike Kottelenberg heard about the results and flat-out didn’t believe them. So they replicated them. Lehrer writes:

Baker, Gruber and Milligan’s work is 100 per cent correct. It’s robust. If anything, in our own work, including a paper that came out in Canadian Public Policy and won “best paper award” for that year, it actually finds the effects get larger over time, on average.

From a public draft of Baker et al, 2019. The different lines show show crime rates go up in cohorts with more exposure to the universal childcare system. The lowest solid line is for children who were not exposed to the system.
  • Most strikingly, the Quebec crime rate goes up by about 20% when the children who were in daycare become teenagers. That figure seems to shock people, but as far as we can tell it’s real. Here’s a way of looking at it. If I told you crime rates were higher among children who grew up in more deprived families, would you be surprised? Typical daycare centers have some similar characteristics. There’s less attention per child, more noise, people are more stressed, etc.. So you get the same effects on brain development and the long-run consequences of that, including in crime rates.
  • The Quebec program covered ages 0–4; the negative findings were disproportionately driven by the youngest children. Hours seem to be long. And the program looks to have mainly changed the behaviour of advantaged families. So it’s not surprising that the outcome was so bad. (Age matters. Hours matter. Income matters.)

Boston

A very recent study looked at a1996 expansion of universal preschool in Boston.

  • The study was only looking at 4-year-olds.
  • Hours were relatively short; at most six hours a day (30 hours a week).
  • The intake was ‘relatively disadvantaged with high shares of non-white and low-income students’.

All three of these are linked to good outcomes. (Age matters. Hours matter. Income matters.) And that’s exactly what the study found—in Boston, preschool improved high-school graduation and college attendance, and decreased juvenile incarceration.

Scandinavia

People talk admiringly about the Scandinavian childcare system, but the research is mixed. (Drugli et al, 2018) find elevated cortisol levels in Norwegian toddlers in childcare; (Berg-Nielsen et al., 2012) note that “sadness, tears and fears may simply be regarded as common and age-appropriate”. (Hårsman, 1984) observed babies in Swedish daycare for five months and reported that 52% of them were “sad and depressed in the day-care setting”.

We really need more research on the Scandinavian system. A psychologist anecdotally recounts being told it would not be funded in case it threatened the Nordic social model.

Policy

You didn’t ask about policies, but I’m going to tell you (briefly) anyway, as the only way to make life better for all our children is at the level of society. Feel free to skip this section.

Disclaimer first. If I tell you guaranteed parental leave makes children better off, you’ll think I’m left-wing. But if I tell you early exposure to/long hours in daycare are bad for children, you’ll think I’m a social conservative. I hate how every issue has become polarised nowadays. It’s why I don’t normally write about this stuff, since arguments and internet flame wars make me miserable. But there are crucial points about childcare that journalists and politicians miss (or choose to obscure). I’m hoping some of you are more thick-skinned than me and can get these facts out there.

  • You sometimes hear that investing in early childhood has huge returns. (The ‘Heckman Equation’.) That’s specifically true for low income children. It’s based on an analysis of two high-quality interventions (1962 Perry Preschool and 1972 Abecedarian). The 200 or so children in those projects were all poor, and nearly all African American.
  • Hours matter. There’s a big difference between being in a center 8am–6pm (50 hours a week) and being in a center 9am-3pm (30 hours a week).
  • Above all else, age matters. Journalists keep missing this. If you muddle together all the evidence from studies on 12-month-olds and studies on 4-year-olds, it looks like the evidence on childcare is mixed. But that’s a completely wrongheaded thing to do.

I understand why many people desperately want to be able to put babies into childcare without any consequences. But that’s like wanting geoengineering to fix climate change without any costs. We live in a world of trade-offs, and in this instance the costs are paid by our youngest children.

What policies leave children and parents better off? For a start, paid leave is fantastic, and stops families having to make miserable choices. It’s a hard political fight as it costs businesses, and they’ll push back hard — but we should fight for it. Pre-K for 4-year olds is also great and may be an easier fight.

There’s also one policy no-brainer which doesn’t cost anyone. That’s to replace government-run childcare programs for babies and toddlers with equivalent cash transfers to parents. (For this to work, the government has to regulate childcare quality because — as noted above — parents can’t measure it.) What would this policy mean for parents?

  • Parents who need to can still spend the money on professional childcare. But because fewer people will use center care (‘reduced demand’), the cost of center care will go down, so parents using it will end up with cash in hand.
  • For other parents, the money will mean that they, or relatives, are able to look after their babies themselves.

So everyone ends up better off. Why would we not do this?

Conclusion

Well, I guess I never answered the question.

Great, so if you were to write a science based, research backed rebuttal to this chapter alone, it would recommend what exactly? No daycare? Daycare in particular family instances at particular ages?

I ask knowing full well I likely won’t be able to meet the ideal, but I personally would prefer to know what that ideal is to attempt to get closer to it.

I hope you now have the information to answer that yourself, because there are a lot of nuances which I’m nervous glossing over. But if you want my own answer, for the average child:

  • The best behavioral and cognitive outcomes come from starting half-days in daycare around 2½. Switching to full days provides no benefits and long days may worsen behavior until around 4.
  • Before 2½, any relative as carer gives the best outcomes. Failing that, nannies are probably better than childminders (in-home daycare) and both are certainly better than daycare centers.
  • All of the negative effects of non-relative childcare are more pronounced for younger children; childcare choices in the first 12 months make the most difference, as children are particularly dependent on their carers then.

Article last revised: November 2021

Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.