A within cultural comparative examination of three samples of school-children residing in Saskatchewan, South Australia and Nova Scotia enabled the effects of family size and socioeconomic status on measured intelligence to be ascertained. Samples of schoolchildren in Regina (Canada), Adelaide (Australia) and Sydney (Canada), divided into subgroups according to family size and socioeconomic status, were tested in the first two samples on the Otis Mental Ability Tests, and in the latter on the Henmon-Nelson Intelligence Test, while socioeconomic status allocation was obtained from the occupation of the father of each child in the three samples. In each sample, the results demonstrated the existence of a definite relationship between intelligence (IQ) and socioeconomic status (SES), in favour of the high SES children. However, the usually reported significant negative correlation between family size and IQ disappeared in the 170 predominantly middle-to-upper SES children in Regina, in the 427 school-children from various SES groups in Adelaide, and in the upper SES group only of the 144 school-children in Sydney (even though the overall correlation for the latter sample was negative and significant). In each sample, a clear indiciation emerged--family size, among middle and upper SES families in Western industrialized affluent societies, generally has no detrimental influence on the mean IQ of its members. Evidence from the Canadian and Australian samples point to the same conclusion--at the least, upper SES children perform equally as well on an intelligence test regardless of the family to which they belong. (Author/JM)