Childcare : what the science says

19 min readMay 2, 2021

I recently wrote about errors in a ‘data driven’ guide to childcare; the first comment asked:

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 didn’t want to get embroiled in the ‘parenting wars’, so promised to DM the commenter. And then this happened…

I am writing this article to avoid fielding requests for DMs for weeks and weeks. It’s for people who have genuine, upcoming choices about what kind of childcare to use. It’s not a commentary on anyone’s parenting choices, and it’s not for people who have to use daycare.

The most common piece of feedback I get is ‘I wish I hadn’t read this’. If you don’t have realistic choices, please stop reading.

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 converge, so that we can be quite confident about findings. If you feel anything is not adequately justified, please let me know here — but please absorb the evidence from outside your own discipline before you do. (Also NB 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 a family of average socioeconomic status (SES).* That term covers the spectrum from poverty/deprivation (lowest SES) up through working class families and then to affluent middle class families (high SES). Children from lower SES families benefit from starting earlier, and children from higher SES families from starting later.

*In earlier versions I wrote ‘income’ to avoid using a technical term, but that was misleading. Researchers often use family income as an easy-to-measure proxy for SES, but child outcomes are affected much more by how much people read to children, etc., than by money. The more enriched the home environment, the later the optimal time for starting daycare.

Age matters. Hours matter. SES 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 SES, 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 SES, see (Melhuish, 2015) and also (Votruba-Drzal, 2004) for cognitive aspects.

(McCartney, 2010) and (Huston, 2015) investigate the causes of the behavioral effect. Watch out for studies based on maternal assessments; those are known to be biased by maternal mental health and stress levels (e.g. Chen, 2017), and were abandoned by the NICHD SECCYD research team in the mid 2000s (Solheim, 2013). Reviews tend to be contaminated by studies with maternal questionnaires, so you really need to look at the original papers here.

The modern classic on SES and parenting styles is (Lareau, 2011). See (Francesconi, 2016) [quoted below] on parenting styles having much more impact than money.

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 young children to daycare will improve their social skills. It doesn’t: the only large study of this finds more time in daycare is linked to poorer social skills in primary school.

A likely reason is that children typically start playing alongside each other around 2 or 2½; playing together starts ~6 months later.* For children under 3, their main social environment isn’t other children; it’s you, their adult carers, who teach them about turn-taking and so on. Daycare ratios mean less adult attention per child, so we’d expect social skills to develop more slowly.

*1-year-olds can play with each other in calm, familiar environments with one-to-one adult support. Very few daycares provide that, though.

From the first textbook on play I had at hand.

With less adult attention in daycare, children learn to interact aggressively instead — a study finds around one incident of physical aggression per 9 mins per child, excluding play-fighting. That in turn likely raises children’s stress levels, which I’ll discuss next.

Sources: The studies mentioned are (NICHD, 2005) on social skills and (Goldstein, 2001) on aggression. (Rubenstein, 1976) found 3% of peer interactions in home settings were aggressive, vs 40% in group settings. The remaining material is very standard, so easier to digest from textbooks like this than papers.

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 high-quality centers (Vermeer 2006). Note the broken y-axis.

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.

Aside: the research focuses on the long-term effects of stress, but I find that too clinical. If we’re making toddlers so stressed that we can measure effects on their behavior five years later, that’s upsetting in and of itself. Plus long-term risk of mental illness worries me more than bad behavior.

A lack of responsive, individual care from adults has been proposed as a cause for the cortisol rise. (That’s not a criticism of daycare workers; it’s just really, really hard for one person to look after 3 or 4 babies at once.)

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. See (Bowlby, 2007) on children’s need for a secure attachment to a responsive adult in daycare settings.

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. Except: 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.

There are, sadly, very few high quality daycare centers in the US. (Helburn, 1995). Also see (NICHD, 2000) for similar data.

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 about 4 places from the middle.

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 that amount. For middle-class children, the effect of long daycare hours on angry/disruptive behavior also falls into this ballpark.

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 average effects are measured. Your child may be affected much less than average, or they may be affected much more. That applies to both the positive and negative effects from daycare — remember age and hours affect the mix of positive/negative outcomes.

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, you need hundreds of data points. It’s (reasonably) feasible to find and engage 200 children in daycare centers and 200 cared for by their mothers. It’s much harder to round up, say, 200 children cared for by nannies. So most research compares center care & 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.

On 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 finds childminders are on par with daycare, but that’s not consistent with other research.) (Dettling, 2000) and (Groeneveld, 2010) found cortisol levels rose through the day with low-quality childminders; (Gunnar, 2010) finds process quality affects cortisol but that structural quality does not. (Structural quality is objective things like caregiver training and experience; process quality is fuzzier things like whether caregivers are warm and caring.)

Other considerations

I get a lot of feedback asking about tradeoffs inherent in childcare choices, especially on family income. It’s hard to say much here as some of you will live in places where childcare is free, while others pay an entire second income for it. This may be useful, though:

Carneiro and Heckman (2003) and Cunha et al. (2006) present evidence that child cognitive and non-cognitive skills diverge at early ages across families with different levels of permanent income during childhood.
Levels of permanent income are highly correlated with family background factors such as parental education and maternal ability, which, when statistically controlled for, largely eliminate the gaps across income classes.
(Francesconi and Heckman, 2016)

That is, most of the difference in child outcomes between the highest and the lowest income families is due to how people parent, not having more money. (That doesn’t mean money is unimportant for child outcomes in all families — near the poverty line it makes a big difference.)

[Heckman is not just a Nobel Laureate, but one of the most influential economists in the world. We’re extremely lucky that he has an interest in early childhood.]

Case studies

I really want to stress that it’s a mistake to say ‘center care is good’ or ‘center care is bad’. It all depends on age, hours and family SES. The following case studies should underline this.

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 (my emphases):

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 crime rate goes up by about 20% when the children who were in daycare become teenagers. That’s consistent with (Vandell, 2020), which traces more time in daycare centers through impulsivity, risk-taking, behavior problems and police contact up to age 26.
The program covered ages 0–4; the negative findings were disproportionately driven by the youngest children. Hours seem to be long. And the program mainly changed the behaviour of advantaged families. So the negative outcomes should not be surprising. (Age matters. Hours matter. SES matters.)

Boston

A recent study looked at a 1996 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. SES matters.) And that’s exactly what the study found—in Boston, preschool improved high-school graduation and college attendance, and decreased juvenile incarceration.

—

Of course, not all studies fit the age/hours/SES pattern perfectly — for example, overly regimented, academic pre-K can be detrimental to four-year-olds (Lipsey, 2018). But age, hours & SES capture the big trends.

Policy

You didn’t ask about policies, but I’m going to tell you (briefly) anyway, as it’s the only way to make life better for all our children. 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. I want to build better environments for all our children, and I’m open to any policies that get us there.

You sometimes hear that investing in early childhood has huge returns. (The ‘Heckman Equation’.) That’s specifically true for disadvantaged 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 parental leave is fantastic. 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.

Massive expansion of high-quality care for younger children is possible but very, very expensive; probably the best childcare system in the world is in Sweden, which spends 2% of GDP on it. Even there, parental leave means few children enter the system before 18 months (OECD, 2006). High quality care for younger children takes so much manpower that nannies/parental leave will be cheaper than center care, due to rent, etc.. A sad political reality is that proposals to ‘expand high quality childcare’ tend to be massively underfunded.

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.