I think the independent variable is the quality of the parents. The book notes a correlation between good, stable parents (ie not those who undernourish their children or have anxiety or other mental disorders) and infants who form secure attachments. In addition, good parents are much more likely to continue to nurture and encourage their child as it grows and make sure they have friends and are happy. Thus although some variation in children may occur, I think the main factor causing attachment and future outcome is the parents.
