This paper summarizes the findings of a series of studies on the effects of "ecological variables" on mother-father-sibling-infant interactions. Under consideration were: (1) the effects of stress on the parental preferences of young infants; (2) the effects of the presence of one parent on the interactions within the other parent-infant dyad; (3) the effects of the presence of one parent on sibling-infant and parent-child interaction; and (4) the effect of a sibling's presence on parent-infant interaction. Analyses of parent-child interaction focused on five affiliative behaviors (smile, vocalize, look, laugh and proffer) and six attachment behaviors (proximity, touch, approach, seek to be held, fuss and reach). For research on sibling-infant interaction, eight behavioral measures were added. Parental vocalization to child was recorded as an index of parental activity. All studies involved at least 20 subjects, at different points in the 7- to 24-month age range. Significance of the stress factor is discussed in light of varying results at different ages. It is indicated that, while mothers are the primary attachment figures of young infants, the fact that distressed infants treat mothers and fathers similarly when only one is accessible, and the fact that infants of certain ages discriminate mother and father from friendly strangers, indicate that fathers are important attachment figures. Findings also indicate that infants from 12 to 24 months of age interact far more with either parent when alone with him or her than when both parents are present. The effects of the entrance of a stranger sibling or other parent on affiliative interaction are also discussed. (Author/BF)