This particular study linking day care and aggression is part of a multi-year longitudinal study performed by a large team of highly respected researchers. Most of the studies produced by this team over the years have been highly favorable to day care. For instance, some studies show that some correlation exists between day care and vocabulary or cognitive development. When those studies came out, people were quick to draw generalizations about all kids being just fine in day care. The truth is that the day-care studies are all over the map.
Some studies show different results for boys than for girls.
Some studies look at particular demographic groups, such as very poor families. I think families need to use the research with a lot of caution and discernment. That leads me to the way in which this research is consistently misused. In my opinion, this kind of work needs to be placed at the service of the family, not at the service of policy wonks. In other words, people seem to want to interpret these studies as giving us some insight into what will work for most kids most of the time, so we can figure out what government should do or not do.