The author argues that environmental variation that occurs within families but is not shared by siblings--nonshared within-family environmental variation--is a major influence on general mental performance and has largely been ignored. Data are presented from longitudinal studies that reveal that intraindividual variation over age in IQ accounts for as much or more variability than has been estimated to be the nonshared within-family environmental variation in IQ. Given these results, the author speculates that such factors may account for 15%-25% of all variability in IQ and 30%-50% of environmental variability in IQ. The implications of this type of environmental factor are considered from conceptual and methodological perspectives.