Cultural evolution is the shifting of the external manifestations of behaviour and artifacts through the manipulation of their basic generative structures in semantic memory. The compound structures of culture arise from the semantic nodes.

The rules of cognitive development determine the manner in which the nodes are created and combined to form the semantic networks - and hence, culture.

These physiological processes impose a strict filtering of stimuli from the environment and alter each step of cognition thereafter, from short-term memory and storage in long-term memory to recall, feeling, reveries, and decision making.

The most fully analyzed example of the biological channelling of culture by the processes of mental filtering and biasing arises in the vocabulary of vision. Light intensity is perceived as a continuum; if the light in a room is raised or lowered gradually with a dimmer switch, the conscious brain perceives the change as a continuous progression along a more or less even gradient. There are no steps or benchmarks, and consequently languages contain relatively few words to describe the variation of light intensity.

In contrast, normally sighted individuals see variation in wavelength not as a continuously varying property of light (which it is) but as the four basic colors of blue, green, yellow, and red, along with various blends in the intermediate zones.

If a room is flooded with monochrome light of short wavelength (blue), and the wavelength is then gradually increased, the change is seen as a series of steps from one basic colour to another. The physiological basis of this illusion is due to the differentiation of the retinal cones into three types, whose maximal sensitivities correspond to blue, green, and red.

The light-sensitive pigments in the cones are membrane proteins, with retinal, a pigment molecule, attached in each case to a protein. When the retinal is altered by a photon of light the apoprotein is reconfigured to activate an outgoing nerve cell.

The red and green pigments have been identified, and the genes specifying them located and sequenced. The Mendelian genetics of colour blindness has also been partially worked out.

The biological constraints in color perception are reflected in languages of all cultures thus far examined. Native speakers of twenty languages around the world were shown arrays of objects standardised by colour and brightness. They were asked to place each of the principal color terms of their language within this two- dimensional array.

The results show that the languages have evolved in a way that conforms closely to the biological processes of colour discrimination. The words fall into largely discrete clusters that correspond, at least approximately, to the principal colours that are innately distinguished.

The existence of a learning bias was further revealed by another experiment conducted by Eleanor Rosch. She exploited the fact that the Dani people of New Guinea have no words to denote colour; they speak only of mill (roughly, "dark") and mola ("light"). If Dani adults set out to learn a colour vocabulary, would they do so more readily if the colour terms corresponded to the principal innate hues? Would cultural innovation be channeled to some extent by the innate genetic constraints?

Rosch divided 68 volunteer Dani into two groups. She taught one a series of newly invented colour terms placed on the principal hue categories of the array (blue, green, yellow, red), where most of the natural vocabularies of other cultures are located.

She taught a second group a series of new terms placed off centre, away from the main clusters formed by other languages.

The first group of volunteers, following the "natural" propensities of colour perception, learned about twice as quickly as those given the competing, less natural colour terms. They also selected these terms more readily when given a choice.

In a parallel experiment on "psychoaesthetics," Gerda Smets measured the degree of physiological arousal in adults caused by geometric designs of varying degrees of complexity. The measure she used was alpha wave blockage, generally interpreted to be an index of arousal even when unaccompanied by conscious awareness.

A sharp peak of maximum response was obtained with computer- generated figures at 20 percent redundancy, the amount found, for example, in a maze with between 10 and 20 angles. Less redundancy and more redundancy were far less stimulating. It does not seem coincidental that 20 percent is approximately the amount of complexity in logotypes, ideographs, frieze design, grille work, and other designs chosen for instant recognition and aesthetic pleasure.

In other words, the development of art and written language may be strongly influenced by an innate constraint in cognition.

The innate bias in learning, described by psychologists as "prepared" (bias toward) and "counterprepared" (bias against), is perhaps most strikingly illustrated by phobias. These are extreme, irrational fears associated with nausea, cold sweat, and other reactions of the central nervous system. It is notable that the phobias are most easily evoked by many of the greatest dangers of mankind's ancient environment, including tight spaces, heights, thunderstorms, running water, snakes, and spiders, but are rarely evoked by the greatest dangers of modern technological society, including guns, knives, automobiles, explosives, and electric sockets.