RACE AND SEX-DIFFERENCES IN HEAD SIZE AND IQ

An analysis of IQ in relation to head size (and by inference, brain si ze) was performed on some 14,000 children and their full siblings, alm ost evenly divided by race (white and black) and sex, on whom data wer e obtained at ages 4 and 7 years in the National Collaborative Perinat al Project. Within each race x sex group, IQ is significantly correlat ed with head size, age and body size having been partialed out. A sign ificant positive correlation between IQ x head size exists not only wi thin subjects (at ages 4 and 7) but also within families and between f amilies (at age 7 only). The within-families correlation (at age 7) is consistent with an intrinsic or pleiotropic correlation between the m ental and physical variables. No significant positive correlation with in families appeared at age 4, despite a significant within-subjects c orrelation at that age. As yet, there are only speculative explanation s of the disparity between the age 4 and age 7 within-family correlati ons of head size with IQ. Although general body size is also correlate d with IQ within subjects and between families, the correlation does n ot exist within families in either age group, which rules out a pleiot ropic correlation between body size and IQ. There are both race and se x differences in head size, although the sex difference in IQ is nil. White and black children who are matched on IQ show, on average, virtu ally zero difference in head size.