Key points are not available for this paper at this time.
Mary D. Salter Ainsworth, Silvia M. Bell, Attachment, Exploration, and Separation: Illustrated by the Behavior of One-Year-Olds in a Strange Situation, Child Development, Vol. 41, No. 1 (Mar., 1970), pp. 49-67
Ainsworth et al. (1970) studied this question.