more similar to each other than any one is to a human brain. What changed since we split off from ou

more similar to each other than any one is to a human brain. What changed since we split off from our ape cousins is 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 spatial analyses, motor behavior, goal directed assessments, and object recognition, and much of this activity is fed forward to the frontal areas for storage in working memory as well as judgment and evaluation. A healthy adult brain is a heavily connected brain. Promiscuity results from a network of interconnected brain regions. Even resting brains show signs of promiscuity. When you lie down in bed and close your eyes, but before you drift off to sleep, your brain — assuming you are an adult and healthy — shows activity in a family of inter-connected brain regions called the default network. This is your brain at rest, but it is anything but at rest. Some of the most active areas involve those that are engaged when we evaluate social relationships, consider what others believe and desire, who they are, and how we might interact in the future. This same default network looks very different in children, as well as in the elderly: it is much less connected. Growing up is connecting up. Growing old is disconnecting. We gain promiscuous thinking as we mature and lose it as we age. If connection is key, then disorders of the mind or physical insult should result in predictable loss of promiscuity. A brain imaging study of individuals with autism is revealing. Individuals with autism fall along a spectrum, from low to high functioning. Though this spectrum captures important Hauser Chapter 1. Nature’s secrets 30 HOUSE_OVERSIGHT_012776