Verbal IQs were calculated for 60 boys who had no siblings (only boys) and 64 who were the first in a sibline (first borns) at two age levels-6 and 7 years versus 11 and 12 years. In addition to a pronounced socioeconomic status (SES) effect, three of four subgroups of only boys obtained significantly higher mean IQs than first borns. These data were interpreted as supporting the view that development of verbal abilities is facilitated by contact with parental models of language behaviour, only boys benefiting from closer contact than first borns. However, interactions among IQs, age and SES suggested that although mere increased contact is adequate to foster verbal development in younger children, quality of parental stimulation is important at higher age levels.