Attachment theory states that attachment is a developmental process about the evolved adaptive tendency to maintain proximity to an attachment figure. The origin of attachment theory can be traced to the publication of two 1958 papers, one being John Bowlby's "the Nature of the Child's Tie to his Mother", in which the precursory concepts of "attachment" were introduced, and Harry Harlow's "the Nature of Love", as based on the results of experiments which showed, approximately, that infant rhesus monkeys preferred emotional attachment over food.[2]