To say that it's better to be smart and cheeky instead of stupid and placid shows that you have no concept of the responsibility having a child entails. So off the children go to the day-care center. I will counter that the majority of families do not need day care. Yes, there are many people who, through their own poor choices or through no fault of their own, do not have a spouse to help them raise a child. For those, day care is a necessary evil, for they have to work to support not only themselves and their children but also the fact that they have to work.
Most families can make it on one salary, contrary to the popular culture.
Some parents don't have a choice about day care. It is, of course, possible that separation from Mom does cause behavior problems. The way to test for that would be a study of identical twins where one twin was randomly assigned to go to day care while the other stayed home with Mom. If such a study turned up meaningful differences, then we could say we'd actually learned something. The perfect summer apparel.