Brief Communication The Mental Arms Race Amplifier

The use o f tools to obtain food is probably a long-standing hominid adaptation. Therefore, the rapid increase in hominid brain size over the last million years cannot be explained by selection for basic tool use alone. 350o alternative theories, here termed the ecological adaptation hypothesis and the intraspecific competition hypothesis, are reviewed and rejected. Instead, a combination of these two theories is offered: the mental arms race amplifier hypothesis.