Another variable that might influence a child's initial attachment to a caregiving figure as well as his future relationships and outlook on life could be a disability that the child grew up with. The initial relationship of a disabled child with his primary caregiver would be of a much different quality than the initial relationship of a child who is not disabled with his primary caregiver. This is because the child's ultimate dependence upon the care of another person would be different in each case. A child without a disability would be expected to be more independent later on in life, and so his or her future endeavors and relationships would be viewed in a much different light.
This is an interesting variable that I had not thought about. A disabled child is left at the mercy of the caregiver regardless of attachment. This would clearly alter the correlation between attachment and potential promising life. This is an example of why we can't determine a causal relationship between one type of attachment and higher well being.