Despite the widespread use of standardized IQ tests to measure human intelligence, problems with such measures have led some to suggest that better indices may derive from measurement of cognitive processes underlying performance on IQ tests (e.g., working memory capacity). However, measures from both approaches may exhibit performance biases in favour of majority groups, due to the influence of prior learning and experience. Mental attentional (M-) capacity is proposed to be a causal factor underlying developmental growth in working memory. Measures of M-capacity index important cognitive variance underlying performance on standardized intelligence tests. These measures appear to be reasonably culture-fair and invariant across content domains. The current study tested theoretical predictions regarding the content-invariance of M-measures and the development of M-capacity for groups of children differing in performance on standardized IQ tests. 91 participants differentiated on the basis of academic stream (intellectually gifted vs. mainstream) and age (grade 4 vs. grade 8) received measures of M-capacity in the verbal and visuo-spatial domains. Children identified as gifted scored about one stage higher on both measures. Results suggest that measures of M-capacity may be useful adjuncts to standardized intelligence measures.