Does nursery daycare harm children? Is there an alternative? Should mothers stay at home? For decades, these questions have panicked parents. In a special investigation, Amelia Gentleman examines the often contradictory evidence

In 1986, a young American academic published a paper where he warned, tentatively, that babies who were looked after in daycare were showing signs of increased levels of aggression and disobedience in later life.

The study unleashed what he describes as a global "firestorm of controversy", and, he believes, did enduring damage to his career. The suggestion that babies suffered by being taken from their parents and handed to carers, usually in nurseries, made uncomfortable reading both for mothers considering returning to work and for policymakers, eager to encourage them back into the workforce.

"I was dismissed as a misogynist who wanted mothers to remain barefoot and pregnant and stay at home. I became a pariah in my own field. I went overnight from a darling young rising star to someone who was just anathema," Jay Belsky, the author of the 1986 study, Infant Daycare: A Cause for Concern?, remembers. He left the US, partly because of the outcry, and is now director of the Institute for the Study of Children, Families and Social Issues at Birkbeck in London. He remains palpably wounded by the response to his original, hesitant suggestion of a link between daycare and behavioural problems in later life.

Professor Belsky's story is an interesting example of the politicised and inflammatory nature of any discussion about nursery care for the under-twos. It is no surprise that parents find the decision about the best care for their babies after they return to work complex and stressful. More unexpected is the degree to which the debate among academics and childcare experts is riven with professional dispute and political sensitivity.

The simple question of whether full-time nursery care for under-twos is good or bad for the child is not simple in the least. Academics and childcare experts not only disagree on the answer, but also on whether we have enough evidence to be equipped even to hazard an answer.

A glance at newspaper coverage of research over the last few months offers a bewilderingly diverse set of conclusions – everything from studies that suggest that nurseries can benefit toddlers, to research that parents can return to work months after the child's birth without any adverse effects, and warnings that excessive exposure to nurseries can result in greater risk-taking (a euphemism for drug and alcohol use) as a teenager.

For parents, who are perhaps already swallowing back feelings of guilt at the prospect of handing over their child, the territory is difficult to navigate.

Beneath the contradictory headlines there is a solid base of evidence that suggests that putting a child under two in full-time nursery may have some adverse emotional and behavioural effects in the long term. These effects are modest and are accompanied by some, equally modest, positive effects on language and cognitive skills. The relatively small statistical impact makes it hard for a parent to assess whether group daycare really poses a significant risk for their child, but even the shadow of a possibility that it might lingers in the mind and makes the decision very fraught.

But it isn't just parents who have to wrestle with this data. The last government worked hard to push mothers back to work, and oversaw a huge expansion in all forms of childcare. In 1981, only 24% of women returned to work within a year of childbirth; by 2001, it was 67%, and the Department for Work and Pensions says that 76% of mothers now return to work within 12 to 18 months of having a child. There are about 15,000 nurseries in the UK, and the number is growing. Around 277,000 children under three are enrolled in day nurseries, and 21% of children aged under two spend some time in day nurseries. After grandparents, day nurseries are the most popular form of childcare for working parents who have children under three.

The new government has yet to set out in any detail proposals for the early years of childhood and on parental leave, but an understanding of this debate will be crucial in determining whether to prioritise daycare or paid leave or childminders.

Although the pile of research has grown over the last three decades, the capacity of the subject to provoke fury and unease has not waned. As one academic explains, when you are talking about "how best to look after the most vulnerable, smallest, tenderest members of society", it is natural that the debate is highly charged.

The controversy over Belsky's hypothesis in the 1980s resulted in the US funding a $100m study that followed 1,000 children from birth to the age of 15 to assess the impact of early childcare on the children later in life.

"Ever since these children were two years of age," Belsky says, "we have found evidence that lots of hours spent in any type of daycare, or lots of exposure to group-based care, across the infant, toddler and preschool years, predicted somewhat more aggression and disobedience in children, somewhat poorer relations with teachers and most recently more risk-taking including sexual and alcohol utilisation, including drugs, and more impulsivity at age 15." These are the findings of National Institute of Child Health and Human Development (NICHD) research, which Belsky has been involved in since its launch. The research also showed that good-quality childcare had a positive effect on cognitive and language development.

Should these findings influence a parent considering nursery care for an under-two? Perhaps not, Belsky says. "Whether we are talking about the good news or the bad news, these are small effects," he says.

"When mothers come to me, and say, 'What should I do with my kid?' I say, 'The truth is these effects are small enough … I don't know if this is a decisive enough finding to tell you what to do with your kid. It's a probability not a certainty. The probability looks small, the effect is modest, not big. You might conclude therefore not to worry about it'."

For policymakers, however, he argues that the conclusion should not be disregarded. "In America today, it is normative for children to start childcare at some point in the first year of life and stay there until they start school. This is the case for over 50% of children," he says.

He continues: "Let's imagine these are small effects. But let's imagine a reception class of 30 children in which two-thirds of them have small effects that make them a little bit more aggressive and disobedient ... versus another class of 30 in which only 10% of them do. Are those teachers going to be doing more time managing and less time teaching? Are those playgrounds going to be less friendly? Are those neighbourhoods going to be affected?

"No one single car pollutes central London or central LA. It's all the cars that do it. People are so ideologically opposed to these findings that instead of being thoughtful about them, they respond as if there is only one way to think about them – small, don't matter, ignore," he says.

He is resigned to the way that parents, policy-makers and fellow academics recoil from his findings. "Anybody who speaks ill of childcare is the enemy – end of story. The guy who first linked Aids with homosexuality back in the early 1980s was accused of being a homophobe. The same kind of idiotic, kneejerk, ideological reaction occurred here. People think I'm against daycare. What I say is, if the weather man says it is going to rain tomorrow, is that because he is against sunshine? People feel very defensive about this area."

Belsky has been accused of catastrophising, and it's probably true to say that not all of his colleagues would agree with his account of his professional victimisation. It is remarkable how scratchy, if not positively disparaging, many of the experts in this field are about their colleagues' work.

Kathy Sylva, another American academic now based here, who has been involved in two major UK studies designed to assess the impact of nursery care on children here, comes to different conclusions. The government-funded Effective Provision of Pre-School Education (EPPE) research, which has followed the progress of 3,000 children since 1996 found that if children had been in nurseries they had raised levels of aggression when they got to school at five, and which were still present at seven. "But by the age of 11, that effect had disappeared," Sylva says, although she concedes that the data could be improved on; the EPPE study began when the child was aged three and tracked retrospectively what the child had been doing in early years.

A second study in 2009, the Family Children Childcare, following 1,000 babies from three months, found no relationship between the amount of childcare experienced by a baby and behaviour problems at 36 months.

Sylva's suggestion that putting your child in full-time nursery care had no negative consequences, was greeted with delight in headlines earlier this summer. "The findings will come as a relief to guilt-ridden parents after a flurry of previous research suggesting a link between early nursery attendance and aggression in later childhood or impaired social skills," the Daily Mail commented.

But her position is more nuanced than that. She says that the research is "very chequered" and that the question is still open to debate. She argues that there are no concerns whatsoever connected with putting a child into nursery above the age of three; between two and three, the research is mixed; and below the age of two, there are some "serious and valid concerns". She is at pains to put those concerns into perspective (pointing out in passing that she gets called by mothers "weeping about the decision"), and argues: "In my view, a child who is securely attached to its parents and who is growing up in a loving, caring home will be more resilient."

"For under-twos, it is a careful decision that has to be taken but there are many, many children in the study who had no bad effects that we could measure," she says. "Children's development is shaped by many, many different factors. If the child is a healthy child, in a family that is supportive and caring and goes to a high-quality childcare setting, the evidence is that the child is not at risk."

Of course, her comments have merely reignited the debate.

Emerging research on the effects of raised cortisol levels have further heightened parental unease about this subject. Cortisol is a hormone released in times of stress, and as it is present in the saliva, it is relatively easy to measure. Cortisol has its uses; in the morning when you wake up it helps to keep you alert; usually during the day its concentrations subside.

A Dutch study from 2006, Children's Elevated Cortisol Levels at Daycare: A Review and Meta-Analysis, concluded: "Our main finding was that at daycare, children display higher cortisol levels compared with the home setting ...

"It was shown that the effect of daycare attendance on cortisol excretion was especially notable in children younger than 36 months. We speculate that children in centre daycare show elevated cortisol levels because of their stressful interactions in a group setting."

A lot of questions remain, though, about the long-term effects, and no longitudinal studies have been done to track this. We still don't know how much cortisol is too much, allowing different childcare experts to treat the evidence with very varying degrees of alarm. Some, such as the child psychologist and Guardian columnist Oliver James, use the evidence to consolidate a strongly held belief that nursery care is not appropriate. His position on putting small babies into daycare is clear. "My advice would be: Don't do it."

"There is no evidence that daycare is advantageous to children from middle-class families," he writes in his most recent book, How Not To F*** Them Up, "and there is considerable evidence that it increases the risk of dysregulated cortisol levels, aggression, disobedience and emotional insecurity, especially if the care is of low quality. Unfortunately, this latter is the norm in the US and the UK."

He continues: "There is now overwhelming evidence that daycare causes children to have abnormal cortisol levels, probably increasing the risk of behavioural problems like aggression, fearfulness and hyperactivity."

"If daycare is as distressing to under-threes as many researchers believe, it would not be surprising if it affected their cortisol levels: when distressed we usually secrete the hormone."

James no longer really dances around his conviction that babies are best at home with their mother, or possibly their father; if that is not possible, he offers a hierarchy of substitution that runs: "Daddy is better than Granny is better than Nanny is better than Minder is better than daycare." In earlier books, he says he was at pains to avoid the controversy that surrounds this issue, and stressed that if mothers felt depressed by staying at home, then childcare might be a better option. In How Not to F*** Them Up, he devotes a chapter to the subject, highlighting in detail his concerns about nurseries for babies. "Let's stop lying about this and pretending it doesn't exist as a problem... it really does. How are mothers going to make good decisions, if nobody is warning them about the real situation?" he says.

James came away from a Woman's Hour discussion of his book in May, feeling that he had been given a very hard time as he set out his argument. Indeed, he made a formal complaint to the BBC Trust about the interview. But Justine Roberts, co-founder of the parenting website Mumsnet, who was also interviewed for the programme, says: "It is all very well saying that nurseries are not good but what upsets people is the suggestion that the solution is for women to take sole care of the baby, not that the answer is to improve nurseries. It is the 'Get her back to the kitchen sink' response that causes such anger."

Sue Gerhardt, the author of Why Love Matters and The Selfish Society, also writes about the effects of cortisol, but is critical of nursery care for broader reasons. She sees them as concepts favoured by parents, for their convenience, reliability and cost, but not geared towards the needs of children.

"I have a lot of clients who say that they are putting their baby into nursery because it needs stimulation, and actually that is just not true," she says. "What babies need is to make an attachment to particular people and they need someone to help them manage their emotions and their feelings and their bodies and how to cope with stress and how to manage themselves. All of this is about being with a responsible person – someone who soothes when you need soothing, who plays when you need it.

"What happens at nurseries is you have the kind of relationship that you have at a party. You meet someone, you do small talk, you might find it pleasant or it might not be. That person isn't really tuned into you. Then you move on to the next person. It is not really a relationship where someone is tuned into you. At nurseries, the turnover is so great that it is very difficult to achieve that kind of relationship," she says.

"More and more people are being given the idea that nurseries are fine for children of any age and that is not the case. It's just about money and convenience."

Penelope Leach's work on childcare has been read by parents for decades. She, too, is concerned by the emerging research on cortisol and is sceptical of governments' preference for nurseries, arguing that they tend to be attracted by economies of scale. She is clear that the advantages of nurseries only begin to be felt when a child progresses to somewhere between the age of 18 and 30 months. Until then, the arguments about socialising the child, giving it stimulation, a head-start educationally, are simply misplaced, she says. "You can tie it to brain development and dawning humanity. When the child is mature enough to understand the difference between morning and afternoon, what people are saying to it, that daddy will come back later – that's the tipping point," she says.

"What I am more concerned about is the first year. We know from attachment theory that if they don't have an opportunity to attach with someone and trust them, then it's a disaster."

On the question of why this area remains so inflammatory, Leach writes in her most recent book that the subject touches on "parents' desperate desire to do the best for their children and the hair trigger of their guilt when that is in question".

"The topic of childcare is becoming more sensitive because, after two generations of startlingly rapid social change and almost a decade into the new millennium, we are still looking at it backward, treating the sole-mother care that was typical of white middle-class families for a generation after the second world war as a gold standard against which to measure (and decry) today's childcare and sometimes look askance at today's parents," she writes. "The reality is that non-maternal childcare is a fundamental part of modern societies; until we recognise that, we shall not recognise, let alone address, the unpalatable reality that much childcare, especially for children under three, is currently of dismally low quality."

For parents making a decision, the evidence can be read simultaneously as reassuring or alarming, depending somewhat on your state of mind. There may be negative effects, but the negative effects are small.

The government's conclusion is: "There is significant evidence on the beneficial effects of childcare for children over two. However, the evidence on the benefits of care for children under two is inconclusive, and it is up to parents to decide what is appropriate for their child. High quantities of centre-based childcare (both hours per week, and a longer duration in months) or very high levels of childminder care at age zero to two years is, however, associated with a small, increased risk of low-level problem behaviours (such as worried and upset behaviours)." The Department for Education statement adds that high-quality childcare can reduce the negative outcomes.

Attitudes shift again depending on whether officials are looking at daycare for working professionals, or at nurseries for children in deprived areas where there is clearer evidence that early intervention can have a positive effect on compensating for family disadvantage.

Professionals in this field who have considered the evidence, offer slightly different advice.

Katherine Rake, chief executive of the Family and Parenting Institute, says we need more long-term studies to assess the impact, but adds that there are not many of these studies because they are so expensive. Alison Garnham, the outgoing chief executive of the Daycare Trust, a childcare campaign group, believes that the jury is still out, and that the evidence is still patchy, pointing out that research highlighting the negative consequences of group care is not accompanied by parallel research looking at the impact of childminders and nannies.

"For children under two, the effects are small: the child might be slightly more bossy, more argumentative – something we might not consider to be negative. In some cultures it might be seen as a good thing," she says.

Purnima Tanuku, chief executive of the National Day Nurseries Association, adds: "There are so many mixed messages out there, and it is always parents who are made to feel guilty as they make these difficult choices."

This debate has been going on since the 1950s when John Bowlby first developed his attachment theory. Although new evidence is emerging every year, the issue in the UK is still far from resolved and is something that is set to cause new parents tremendous anxiety.

In the US, however, Belsky senses a waning interest in the theme – not because the issue is settled research-wise, but because nursery care for the very young has become so accepted that people no longer question it.

"Back in the 80s and 90s, people were wrestling with this. Now it has already been decided," he says. "It doesn't matter what the research says, this is the direction people are moving in – more care at younger ages. We've moved on."