Second, even though glial cells are at least as numerous as neurons in the human brain20, gene sets related to glial cells (astrocytes, myelination, and positive regulation of gliogenesis) are absent from those identified as positively enriched (Supplementary Table 8). Furthermore, using stratified LD Score regression21, we estimated relatively weak enrichment of genes highly expressed in glial cells (Supplementary Table 9): 1.08-fold for astrocytes (P = 0.07) and 1.09-fold for oligodendrocytes (P = 0.06) versus 1.33-fold for neurons (P = 2.89×10−11). Because myelination increases the speed with which signals are transmitted along axons22, the absence of enrichment of genes related to glial cells may weigh against the hypothesis that differences across people in cognition are driven by differences in transmission speed.
