In the Premises chapter chapter we set out some normative design rules for user interfaces. In this chapter we'll ground the most important of these rules in what is known about human cognitive psychology.^[25]

Human beings are not designed for dealing with computers, so computers have to be designed for dealing with human beings. Effective user interfaces have to be designed around the cognitive capabilities that human beings evolved to cope with what evolutionary biologists refer to as the “environment of ancestral adaptation”, the East African savannah on which our ancestors hunted, mated, fought, raised young, and died for millions of years before newfangled inventions like fire, the wheel, writing, and (eventually) computers bubbled up out of the hominid forebrain.

Looking back at the evolutionary story can help us organize our understanding of what we have learned about the capacity of human beings to think, remember, and pay attention. These capacities have many curious quirks, limits and nonlinearities that are profoundly relevant to UI design. Those quirks are much easier to understand if we realize that the kind of logic-intensive thought that computers call on is a lately-developed add-on to an elaborate framework of pre-logical instincts and wired-in responses, one more adapted to coping with saberteeth than city life.

At the end of the chapter we'll discuss ways in which human social instincts can be co-opted to make the design of effective UIs easier.