The present study empirically explored the origins of infant's attachment to a “transitional object” (TO)(Winnicott, 1953), such as a blanket and a soft toy, especially through examining the effects of birth-order, birth-spacing, sex, temperament, etc. on the incidence of TOs. Data were collected from 951 mothers of normal children aged 1-6 years. As for two and three siblings, birth-order effects were not significant, but the frequency of TO attachments among the only children without siblings was significantly higher than that of children with siblings. And the occurrence of TO attachments among first-borns showed a significant tendency to coincide with that of later borns. As for two siblings, birth-spacing effects were significant, such that the frequency of TO attachments among second-borns tended to increase with decreasing birth-spacing from first-borns. In addition, children with TOs were significantly lower than children without TOs in threshold of responsiveness as a temperamental characteristic. Theoretical implications of these results were considered in terms of some stress within mother-infant interactions as a determinant of the occurrence of TO attachments.