Attachment theory is a psychological, evolutionary and ethological theory that provides a descriptive and explanatory framework for discussion of interpersonal relationships between human beings. Attachment theory was developed by John Bowlby as a consequence of his dissatisfaction with existing theories of early relationships. Mary Ainsworth's innovative methodology and comprehensive observational studies informed much of the theory, expanded its concepts and enabled its tenets to be empirically tested. In Bowlby's approach, the human infant is considered to have a need for a secure relationship with adult caregivers, without which normal social and emotional development will not occur. However, different relationship experiences can lead to different developmental outcomes. Mary Ainsworth developed a theory of a number of attachment styles in infants in which distinct characteristics have been identified known as secure attachment, avoidant attachment, anxious attachment and, later, disorganized attachment. Subsequently other theorists extended attachment theory to adults. Attachment styles can be measured in both infants and adults, although measurement in middle childhood is problematic. In addition to care-seeking by children, attachment behaviors include peer relationships of all ages, romantic and sexual attraction, and responses to the care needs of infants or sick or elderly adults.