more similar to each other than any one is to a human brain. What changed since we split off from our ape cousins 1s both the overall geometry of the brain in terms of the relative size of different components, as well as the connections both within and between these components. Some of the most spectacular changes evolved within the frontal and temporal lobes, as well as their connections to other areas of the brain involved in the control of emotion and stress. These circuits play a critical role in decision making, self control, short-term memory, social relationships, tool use and language. For detail, and further evidence of the importance of connectivity in promiscuous thinking, we turn to brain imaging studies of healthy adults, developing children, and patient populations that lack the signature of promiscuity. Consider tool use. Though a wide variety of nonhuman animals use tools, only humans create tools that combine different materials, have multiple functioning parts, can be used for functions other than the one originally designed, and function in the context of survival, reproduction, and leisure. These properties are the signature of a promiscuous brain. When we look at the material culture of the most sophisticated animal tool user — the chimpanzee — we see tools that use a single material, have only a single functional part, are only designed for one function, and the function set is strictly limited to survival or reproduction. Something as simple as a pencil, beyond the chimpanzees’ wildest imagination, consists of multiple materials (rubber, wood, lead, metal), was designed for writing but can be used for poking or keeping hair up in a bun, and has two functional parts (lead for writing, rubber for erasing). When you put a human subject in a brain scanner and record activity during observations of tool use, what you see is an orchestrated coordination between different and connected brain regions. There is activity in regions carrying out spatia