Summary. The intra pair differences in intelligence test scores and measures of physique were correlated for two samples of twins. In both the correlation coefficients so obtained were not significantly different from zero. The method of correlating intra pair differences is equivalent to holding socio‐economic differences constant, as each pair of twins has the same socio‐economic environment. The findings of this inquiry are compatible with the hypothesis that the observed correlations between measures of intelligence and physique, which are positive and greater than zero, are due to common environmental influences, and that there is no biological correlation between mental and physical growth.