Momie, 'all the studies' disagree on when it is 'best' to send a child to childcare, because it depends on:
Parenting
Quality of childcare
Length of day
Child's disposition
Adult/child ratios
and many other things besides.
The majority of studies into this are Scandinavian (children who have been in full-time childcare have enhanced abilities to make good friends amongst their peer group, even years later); US (children from low income homes do better educationally and socially if they attend high quality daycare), and the UK (same findings as US with some findings about children from low income families who attend poor quality childcare settings being nervous about when they will be next fed, being left to cry too much etc). Research focuses on different things depending on the political priority of any particular country at any given time. In World War II, for example, their were suspiciously few studies on the suitability of creches for ammunition factory workers, but by the 1950/1960s, when they did not need women in the workplace so much, and state nurseries had all but closed, suddenly we started getting studies on attachment theory and so on.
In other words, suitability of childcare depends on a number of complex factors that vary greatly from family to family and child to child, and are located in history/politics.
However there is a general and pretty enduring view in the many and various studies that if kids aren't out of the house at a group a few times a week by the age of about three, getting to know how the world works and how to interact with other people, then this is not a great idea, and parents ought to look out some provision so their kids can expand their horizons a bit. However, it is a rare parent that plonks their toddler in a bouncer or on the sofa in front of the TV all day at the age of three, never going out or interacting much. The children of such parents do feature in reports are they are behind in speech when they start school, etc. They tend to crop up particularly noticeably in deprived areas, for example coastal towns (I am thinking of a recent study that looked at kids in Norfolk, for example).
In terms of what is important for the average MNetter, it's really quite straightforward. Parents should do what's best for their family without fretting endlessly about the minor details. No child is going to be damaged by being in a perfectly normal play facility with trained people and colourful posters on the walls and fluffy toys all over the place for two hours a week with other kids, waiting for mum to come back.
Seriously.