Childcare : what the science says
Tags: Childcare, Parenting, Babies, Children, Daycare

Urk. 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’ve probably found this article by chance, and already 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

The Emily Oster approach to science is…

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. You shouldn’t believe me, Emily Oster or anyone else, because we claim to have credentials or because we throw in technical terms and a few vague citations. The key to science is evidence, and particularly backing up specific assertions with links to experimental evidence. Even if asked for my own recommendation, I’m not comfortable giving it without adding enough sources that you can check on me.

Academic articles include their sources using extensive inline citations and technical language. That looks impressive and intimidating, and I’m more used to it than to writing like this — but it’s terrible science communication, as it obscures the forest in the trees. I’ll instead break this article into sections, and give sources after each section. Those won’t be comprehensive, for space reasons, but there will be enough for you to check on what I’m saying.

Last caveat: research looks at average effects on large numbers of children. (Because statistics.) Your child is very possibly not average. I’m not going to write ‘on average’ in front of every sentence below— but, please, take it as read.

Okay, onto the bit you’re here for!

Daycare Centers

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.

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.

, 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.

, 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. There is no boost to social skills.

, 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. 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.

Compared to 15–30 hours, 30+ hours a week before 4½ 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 large numbers hours in any kind of out-of-home childcare have been found to be three times as likely to have “elevated levels of aggression”. (I’ll say more about effect sizes later.)

Note the age and long-hours effects are additive: putting 6-month-old children in daycare for long hours has a particularly large negative effect on later behavior. (Strictly speaking, there’s an ‘interaction term’: [young and long hours] is worse than [young] plus [long hours].)

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 it correlates with — but income is easy to measure objectively so researchers measure it.) The most deprived children can actually benefit from starting as 1-year-olds.

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). 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:

Parenting quality impacts cognitive skills more than daycare, but daycare impacts behavior somewhat more than parenting. 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.

impacts cognitive skills more than daycare, but daycare impacts behavior somewhat more than parenting. 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 quality. 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, which 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

The next question is, why does daycare usage affect 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 the levels of cortisol, a stress hormone. This is used extensively outside childcare research; there are many studies showing that e.g. growing up in adversity or prenatal substance abuse permanently affect the cortisol response and that that shifted response 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.

You’ll note 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. Similarly it turns out daycare has a larger effect on cortisol in children under 36 months; this fits with the finding that they are more likely to have behavioral problems later on. (Very young children do not exhibit rising cortisol but likely have not formed consistent cortisol patterns/sleep patterns. As you may have experienced… .) Higher quality settings cause smaller cortisol rises, which fits with their having smaller effects on behavior (as discussed below).

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 small children so stressed. It was originally thought that the general chaos, noise and exposure to large numbers of infants might cause it. This turns out not to be the case: cortisol rises similarly 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 is the meta-analysis/review (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".

When friends ask me about childcare, they always say that they’ve personally been lucky and found fantastic settings. The sad thing is that researchers have found that nearly all parents rate their settings as great. In fact, 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, you’re best off starting with general reviews. There’s a very readable (if outdated) summary on p3–4 of (Bradley, 2007) and a more thorough account in (Burchinal, 2015). If you want to be really certain that you can assess quality, the professional assessments are supported by this organization.

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 by unscrupulous people trying 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 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.

; 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 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.)

(a.k.a. in-home daycare providers) can cause later behavioral problems, but much less so than . 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.

Also, I’ve left out some topics just to keep this article at a readable 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 how gender affects everything — 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 than girls to start with.* I have never had the time to look into that properly. More generally, I have tried to limit this to findings that a) have been replicated enough to be reliable and b) are useful to parents. If you want to go beyond that, the reviews above should start you off; let me know if you have trouble finding anything.

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

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 probably think I’m left-wing. But if I tell you early exposure to/long hours in daycare are bad for children, you’ll probably 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 I hate arguments and internet flame wars and so on. 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.

Journalists love saying that investing in early childhood has huge returns. That’s only true for low income children. It’s not an argument for subsidising childcare more generally.

Quebec implemented universal childcare in 1996. The results were… catastrophic. Just as you’d expect from the science covered above, the children involved ended up with no lasting cognitive benefits and also a ‘significant decline in health and in life satisfaction’ and a ‘sharp and contemporaneous increase in criminal behavior’ later in life. The crime rate goes up by about 20% when these children become teenagers (Baker et al, 2019).

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.

That 20% figure seems to shock people, but as far as we can tell it’s real. The paper uses methods that tease out causation, not correlation, and is extremely careful with statistics. 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, and so on. So you get the same effects on brain development and the long-run consequences of that, including in crime rates.

figure seems to shock people, but as far as we can tell it’s real. The paper uses methods that tease out causation, not correlation, and is extremely careful with statistics. 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, and so on. So you get the same effects on brain development and the long-run consequences of that, including in crime rates. People talk admiringly about the Scandinavian childcare system, but the research is mixed on this. (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 emotions for 4-year-olds in Norwegian daycare settings.” (Research is generally scarce; a psychologist anecdotally recounts being told it would not be funded in case it threatened the Nordic social model.)

I understand why many people desperately want to be able to put babies and infants 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 children.

There is one policy no-brainer which doesn’t get enough attention. Subsidising childcare is always worse than equivalent cash transfers to parents. Parents who choose to can still spend the money on professional childcare (same outcome), but the money will enable parents who prefer to look after their children themselves do so (better outcome for parents and children). Actually, if you subsidise childcare for everyone, that massively drives up demand and the then underlying price of childcare goes up, so even parents who prefer childcare are worse off. Plus the massive expansion of childcare means the quality drops, resulting in the Quebec debacle.

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: