The mechanism underlying mirrored activity/movements in normal individuals is unknown. To investigate this, we studied 11 adults and 39 children who performed sequential finger-thumb opposition or repetitive index finger abduction. Surface electromyographic (EMG) activity recorded from the left and right first dorsal interosseous muscles (1DI) during unilateral sequential finger-thumb opposition (voluntarily activated muscle, 1DIvol) showed mirrored EMG activity (homologous muscle of the opposite hand, 1DImm) that decreased with increasing age. The time of onset of involuntary compared with voluntary EMG activity was variable but could start at the same time. A significant increase in E2 (transcortical component) size of the cutaneomuscular reflex recorded from the 1DImm indicated increased excitability of the motor cortex ipsilateral to the 1DIvol during active index finger abduction compared with the 1DIvol relaxed. Transcranial magnetic stimulation, using the Bistim technique, indicated that the transcallosal inhibitory pathway in children may not operate in the same way as in the adult. Cross-correlation analysis did not detect shared synaptic input to motoneuron pools innervating homologous left and right hand muscles. We conclude that the mirrored movements/activity observed in healthy adults and children are produced by simultaneous activation of crossed corticospinal pathways originating from both left and right motor cortices.