There are many issues here. Pofaced, thanks for sending us that Guardian piece.
First issue: very predictably, in the guardian piece there is not one- NOT ONE- mention of fathers. Also, there is the assumption (just thrown in, casually) that 'secure attachment to mother' is the number one important issue. Well I disagree completely with this. Even in the earliest John Bowlby literature, it never says that attachment to mother is the most importnat issue. Attachment to 1-2-3 important figures is the key. A good nanny could play this role. A father too. A grandmother. Etc.
Second issue. There is a general talk of 'nursery'. But what are we talking about here? surely there are differences between a 3 month old going to nursery- a 2 year old going to nursery- a 3 year old going to nursery- and also differences between part time nursery care & full time nursery care. And also differences in quality of nursery care. All these different scenarios are not really discussed, in their individuality & complexity. Plus of course the HUGE differences in family circumstances.
Third issue. Pofaced, you say (rather arbitrarily, I feel) that nursery is best 'after the child is 3'. In order for this to happen, one or both of the parents need to stay at home full time to take care of the child (unless they can afford a full-time or part-time nanny). This in turn means that most likely the woman will be doing the SAH, and it also means that she will definitely be left behind in her career, her interests. Plus she may WANT to return to the work for which (in the case of many women) she has prepared for years & years. The work which in many cases means not only money but a feeling of identity too.
The issue of nursery (and the focus on mothers) is for me a red herring. The main issues are different. Why isn't there a focus on fathers' and mothers' responsibility to their children? Why do we keep arguing about these issues- we mothers- instead of pushing for more flexible work, more childcare in the work space, more affordable childcare, longer maternity & paternity leave etc etc?
As Bellsa said, I think nursery (or other childcare) can be an excellent choice for some particular children & families. It's silly to assume that all mothers would necessarily be good as SAHMs- not all parents are good at the baby stage or enjoy it- or that their children would necessarily thrive & be at their best with a SAHM, if the mum herself doesn't feel comfortable in that role. As I said, the issues are much wider & all this supposed research is beside the point IMO (plus, it doesn't seem like convincing research at all to me, all this talk of 'aggresivity' & what it means is, I think, very culturally specific).