Despite many observations of anterior cingulate cortex (ACC) activity related to cognition and affect in humans and nonhuman animals, little is known about the causal role of the ACC in psychological processes. Here, we investigate the ACC’s causal role in affective responding to threat in rhesus monkeys ( Macaca mulatta ), a species with an ACC largely homologous to humans in structure and connectivity. Male adult monkeys received bilateral ibotenate axon-sparing lesions to the ACC (sulcus and gyrus of areas 24, 32, and 25) and were tested in two classic tasks of monkey threat processing, the human intruder and object responsiveness tasks. Monkeys with ACC lesions did not significantly differ from controls in their overall mean reactivity towards threatening or novel stimuli. However, while control monkeys maintained their reactivity across test days, monkeys with ACC lesions reduced their reactivity towards stimuli as days advanced. Critically, this attenuated reactivity was found even when the stimuli p...