adults who are sensitive and responsive in social interactions with the infant, and who remain as consistent caregivers for some months during the period from about 6 months to two years of age. Parental responses lead to the development of patterns of attachment which in turn lead to "internal working models" which will guide the individual's feelings, thoughts, and expectations in later relationships. More specifically, Bowlby explained in his three volume series on attachment (1973, 1980, & 1982) that all humans develop an internal working model of the self and an internal working model of others. The self-model and other-model are built off of early experiences with their primary caregiver and shape an individual's expectation on future interactions with others and interactions within interpersonal relationships. The self-model will determine how the individual sees themselves, which will impact their self-confidence, self-esteem, and dependency. The other-model will determine how an individual sees others, which will impact their avoidance or approach orientation, loneliness, isolation, and social interactions. 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. As the toddler grows, it uses its attachment figure or figures as a "secure base" from which to explore. Mary Ainsworth used this feature in addition to "stranger wariness" and reunion behaviours, other features of attachment behaviour, to develop a research tool called the "strange situation" for developing and classifying different attachment styles. The attachment process is not gender specific as infants will form attachments to any consistent caregiver who is sensitive and responsive in social interactions with the infant. The quality of the social engagement appears to be more influential than amount of time spent. Darwin biography Bowlby's last work, published posthumously, is a biography of Charles Darwin, which discusses Darwin's "mysterious illness" and whether it was psychosomatic. In this work, Bowlby explained that: Bowlby pointed out that Darwin suffered a curious denial about his mother’s death, once writing in a condolence letter “never in my life having lost one near relation” and during a Scrabble-like game when another player added ‘M’ to ‘OTHER’, he stared long at the board then cried: ‘There's no such word as M-OTHER’. Bowlby's legacy Although not without its critics, attachment theory has been described as the dominant approach to understanding early social development and it has given rise to a