The other answer gave really good research background into this, but please keep in mind something when reading this research:
- The sample set is often small (a few daycare/children surveyed)... For example, the "large scale UK study" that the other answer linked surveyed about 800 kids in 100 different daycares. Which sounds like a lot until you realize that there are 3.9 million children under the age of 5 in the UK, so the study surveyed a total of 0.0205% of the children (probably a bit more since it focused on kids 20-42mo). Either way it is hard to draw a hard-line conclusion from such a small sample set.
- "Social-ability" or "anti-social-ness" is not an empirical measurement. Kids behavior changes with the wind.
- Usually these studies do not survey a wide range of socio-economic backgrounds. For example they may only survey one or two daycares in a predominately one-race/economic area.
- These studies usually do not follow the child through development, and children at that age especially develop socially at different rates and with different "stages". These studies are simply a "snapshot" in time, not a long term study.
- They don't take into account the childs home-life. If they have parents who fight they will mimic that behavior, same with older siblings or broken families.
That being said all I have left is personal experience. Our children (now age 4 and 5) have been in daycare since the age of 8 weeks. They went through phases in daycare that would have branded them "anti-social" if surveyed at the right time:
- Preferred to play alone on occasion
- Demonstrated tendencies to bite, kick, hit, etc. the classmates
- General disobedience for directions given by the instructor
That being said, kids go through those phases when they are learning how to interact with peers on a social level. They don't have the skills to say "no thank you, not right now" so they lash out with biting/violence. They are testing boundaries so they may disobey. They are adjusting to schedules/nap times so they get cranky. They prefer to experience a toy or activity without having to share, so they play alone...
My personal view is that having these interactions helps them cope with navigating a social environment. They are gaining skills to read a situation and respond accordingly and appropriately. They are learning what boundaries are and what consequences mean. This does not mean they are anti-social, they are simply learning how to "fit-in" and still be individuals.
Again, my own children went through those phases and they can last anywhere from a couple weeks to many months. Once they are ready to move out of that, they stop exhibiting those behaviors and learn to fit in with their peers much better.
Now, both of my children have no problem becoming "best friends" with peers that they meet at a playground for the first time. It's actually quite humbling that my kids can run up to somebody new, somebody they observe for a few minutes, and determine that they can interact with those in a way that is fun for all of them. This is something we lose as adults!
On the flip side I'm not so sure that a child that is kept in the company of close friends/family can have those same experiences. Kids that are kept home and even home-schooled grow up to be normal functioning adults. I'd venture to say that trying to say kids are anti-social if they are in daycare vs those who are not may be splitting hairs, especially given the sample size and longevity of the "study".
My children (and many of their peers) are in daycare 40+ hours a week, and this is quite normal nowdays. We live in a society where both parents need to work to be able to afford a home, food, and entertainment for the family. Nobody should ever feel ashamed to need to put their kids in childcare so that they can provide for their family, and no parent should worry about the social outcome of the children that are there.
If you have a bad experience with a daycare (and there are good and bad ones), try moving your child to a new place (but not too often). It may take them time to adjust but you will notice if the staff truly cares about your child and their emotional development.