This paper discusses the differences between the one-child family and the family with more than one child in terms of the effects or possible effects on the child. Longitudinal studies of 200 children being followed from birth through age 5 (one-third of whom are only children, two-thirds, second children, were used as a data base, along with a less systematically collected data base on only children and their parents and a comparison group of non-only families. Certain psychodynamic features that seemed to relate to the composition and structure of the only child family were identified. These included: (1) increased parental focus on the only child, resulting in greater mutual dependence; (2) heightened ambivalence between parent and child generated by their emotional and physical closeness; (3) multiple parent roles, since parent often acts as a substitute for the absent sibling; (4) adult-like adjustment (the child has to adjust to an adult-oriented household); (5) identification with parents (the models that parents provide are very difficult for any child to identify with successfully, so the only child often feels inadequate); (6) values (emphasizing respects for individuality and independence rather than sharing and cooperation) which do not necessarily facilitate his getting along with other children; (7) parental expectations which encourage cognitive and intellectual independence but not necessarily emotional independence; (8) recipient of parental problems (the only child is much more at the mercy of what happens within the family); (9) power structure, which can often result in two-against-one situations. (MS)