On one side, there are experts like Reiner Boehm who are openly opposed to the early daycare practice. In his works, Boehm cites a research document from the National Institute on Child Health and Development which spans 15 years of examination of the mental and behavioural development of 1300 children. What it found was that in their teenage years, children who have been subjected to early daycare, are exhibiting a number of negative effects on their social and emotional competence compared to their peers.
Boehm cites "glaring behavioural deviations", like extraordinary rates of smoking addiction, alcohol consummation, drug use, criminal behaviour and vandalism. And all that, regardless of the quality of the daycare institution that the child has attended. Further, more recent research where children of very early age were examined for their daily levels of the stress hormone cortisol, have shown levels "comparable to those of stressed managers in big companies". The chronic stress of course disrupts brain development and affects behaviour in adulthood. The first two years of the child's development are especially delicate in this respect.
Meanwhile, the opposite camp in this debate includes experts like Katharina Spiess of the German Institute for Economic Research in Berlin, who is critical of Boehm's assessment and cites a Michigan research of children at preschool age (the Perry Preschool Project), a long-time project that started in the 60s and examined the development of 123 African American children from poor families who started attending half-day nursery at a very early age. The conclusion was that "this had a tremendous positive effect on the children's development". Compared to their peers who hadn't been to early daycare, those (now 50 year old grown-ups) are earning higher income, have created families and have exhibited criminal behaviour on very rare occasions.
Boehm is also criticised by Yvonne Anders of the University of Bamberg who cites the long-running British EPPE Project which began in 1997 and included 3000 children, and which eventually demonstrated the positive effect on social behaviour and success rates at school for children who had attended early kindergarten. The conclusion is that, no matter if a child has been raised at home or elsewhere, the family environment, the parents' background, the living conditions at home are by far the most important factor for the child's development, outweighing and overshadowing any other external factor - such as the timing of daycare.
And of course, there is also the moral/practical question whether nursery doesn't serve the interests of the adults rather than the children, allowing the parents to reconcile their career with family duties at the expense of the child's adequate development. The debate is still open, so I'm inviting the audience to throw in their respective anecdotal examples. Let's compare notes!