Two competing sets of genetic instructions direct the brain’s cortical development in multiple species, a new study finds. One influential explanation—the tethering hypothesis—proposed that primates decoupled the association cortex from the developmental constraints that shape sensory regions. As the primate cortex enlarged during evolution, sensory and motor regions remained relatively fixed, while the association cortex occupied the remaining territory. Rather than becoming untethered from primary sensorimotor areas, association areas actively compete with these regions to establish cortical terrain. Sestan and his colleagues discovered the two competing developmental programs in cortical samples from fetal human and macaque brains of varying developmental stages.