Another alternative explanation can be the level attachment among a baby's own family members. Depending on different societies(cultural backgrounds), physical distance between family members, and traditions, the attachment strength among family members varies tremendously. Some families keep much closer relationship among themselves compared to some others by socializing more frequently, helping each other more often, etc. A child growing up looking at how close his sister is to his parents, for example, learns different types of attitudes. And the modeling process affects attachment style and well-being later in life.
