One alternate explanation for why children with secure attachments also do well later in life could be that children who are able to secure attachments have personalities which allow them to be secure with themselves, self-confident, and optimistic (that is, having the belief that one can succeed). These traits are beneficial for a person because with self-confidence, a person is more willing to approach others and befriend them (thus be more popular, which is discussed in the text), to be motivated to do well (has the internal belief that they can succeed), and to stand up for oneself. This has nothing to do with the secure attachment, but rather comes from the innate traits in the person that allowed him to form a secure attachment in the first place.
