Something many fail to appreciate today is that Bowlby was very much a radical thinker in his day, emphasizing as he did the role of "lived" experiences in shaping development. The prevailing theoretical tradition of psychoanalysis of the time stipulated that it was the child's fantasy world, not his actual reality that shaped his well being. Radicals like Bowlby were not embraced; they were rejected. Thankfully, history is the final arbiter of these matters and so psychology today is very much steeped in Bowlbian ways of thinking.
Bowlby's colleague, Mary Ainsworth, initiated the scientific study of rearing influences on early attachment security to test his radical premise. And ever since reporting her work in the 1970s, a cottage industry has developed evaluating her hypothesis that it is sensitive mothering--or caregiving--in particular that fosters security in the infant and young child. Infants establish secure attachments when caregivers, be they mothers, fathers, or even child-care workers, recognise the infant's signals (verbal and nonverbal), accurately interpret them and respond in a timely fashion and an appropriate manner. Responding to a 6-month old who is holding his hand out, showing a toy with a smile and appropriate comment ("what have you got there?") could be an instance of sensitive responsiveness, but not if it occurrs so long after the event that the infant is unlikely to link the two together. Hugging, kissing and showing affection to a young child could be sensitive, but not if it abruptly interrupts on-going exploration and play. And, of course, comforting a crying child could be sensitive, but not if it is accompanied with a verbal barrage criticising the baby for being such a pain.
It appears that some children are simply born secure, whereas others are made secure or insecure by, as theory would have it, the quality of rearing they experience. Thus, the reason why effects of sensitive-responsive parenting on attachment security may not have proven as large as expected is because studies have (unknowningly) mixed apples and oranges together, the apples being the children who are not affected by their rearing and simply born secure, or so it seems--lucky them; and the oranges being those for whom quality of rearing does matter. Mix the two together and you water down the effect of rearing that would otherwise be detected for the second--orange--group.
What distinguishes those who seem influenced by the care they receive, at least with regard to attachment security, and those who do not? Genetics! In yet another outstanding piece of research, Grazyna Kochanska and her colleagues at the University of Iowa have discovered that infants with one or two short alleles on the serotonin transporter gene(5-HTTLPR) established secure attachments to their mothers when their care was sensitively responsive, but developed insecure attachments when it was not. Those with only long alleles on the same gene were highly likely to establish secure attachments even when they experienced care that was insensitively unresponsive; these were the children whom I claimed were "born secure". (See: http://www3.interscience.wiley.com/cgi-bin/fulltext/121421420/PDF...)
Why this particular gene? The exact answer to that question is not quite clear. It is known that this gene is related to the expression of negative emotion, so babies with short alleles and thus more prone to negative emotion may need more assistance in developing the capacity to regulate those feelings; and developing the ability to do that, via sensitive care, could be what promotes attachment security. Infants with the long alleles simply don't face this, or as much of a challenge.
Of interest is that short alleles on this gene have been found to make women more likely to become depressed if exposed to many negative life events. Women with long alleles seem very much protected from depression even if they confront the same negative life events.
In other words, and to return to a theme I have championed in earlier blogs, children--and adults--seem to vary in their susceptibility to environmental influences. This raises the prospect that one resolution to the long-standing nature-nurture controversy involves the following: For some who are very intelligent or highly aggressive or lethargic or not particularly verbal, this is because they are born this way. For others, who are exactly the same, this is because they have been made this way.