Key points are not available for this paper at this time.
Eleanor E. Maccoby, S. Shirley Feldman, Mother-Attachment and Stranger-Reactions in the Third Year of Life, Monographs of the Society for Research in Child Development, Vol. 37, No. 1, Mother-Attachment and Stranger-Reactions in the Third Year of Life (Mar., 1972), pp. 1-86
Maccoby et al. (1972) studied this question.