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H. Rudolph Schaffer, Peggy E. Emerson, The Development of Social Attachments in Infancy, Monographs of the Society for Research in Child Development, Vol. 29, No. 3, The Development of Social Attachments in Infancy (1964), pp. 1+3+5-9+11-19+21-41+43-61+63-73+75-77
Schaffer et al. (Wed,) studied this question.