IF SOMEONE SHOWED you a caricature of Richard Nixon—a man’s face with oversize shaggy eyebrows, a bulbous nose and pronounced jowls—you would probably recognize the former president immediately, even though the drawing is not true to life. A cartoonist creates such a sketch by taking the average of many male faces and subtracting it from Nixon’s face, then amplifying those distinctive differences. To an observer, the result looks more like Nixon than Nixon himself. Why is it that our brains respond so intensely to extremes?
When the cartoon’s “Nixon-ness” jumps out at you, you are experiencing what scientists call “peak shift.” To understand the concept, imagine, for argument’s sake, that you want to teach a rat to distinguish a rectangle from a square. It’s quite easy to do. Simply give the animal cheese every time it picks the rectangle, and it will soon learn to select the rectangle every time. Once the rat has developed this preference, let’s say you show it a longer, skinnier rectangle. Inevitably, you will find that the rat prefers the exaggerated one to the original. What the rat has learned to recognize is not a particular rectangle but rather rectangularity itself: the more rectangular the better. The savvy rodent looks at the longer, skinnier quadrilateral and goes, “Wow, what a rectangle!” In scientific parlance, the rat’s “peak response”—its strongest reaction—has shifted away from the original—hence the term “peak shift.”
The sway that exaggerated characteristics hold over us is a special kind of illusion—and a powerful one, we believe. In the five years that we have been writing about perception for this magazine, we have described a range of illusions, from geometric patterns that seem to move because they activate our motion perception systems to optical tricks that arise because each of our eyes sees the world from a slightly different position.
Now we would like to make a daring suggestion: that illusions are not merely fascinating windows into our minds and the way we perceive the world. They help to drive the most powerful force that shapes life on earth: evolution.
The standard theory of evolution is that animals that randomly inherited genes that produced beneficial traits—in the case of the giraffe, a longer neck, which made it easier to reach tall acacia trees—ate better, reproduced more often and passed these gene variants to their offspring. Hence the progressive lengthening of the giraffe’s neck across successive generations.
What we are proposing is yet another mechanism of evolution. Our hypothesis involves the unintended consequences of aesthetic and perceptual laws that evolved to help creatures quickly identify what in their surroundings is useful (food and potential mates) and what constitutes a threat (environmental dangers and predators). We believe that these laws indirectly drive many aspects of the evolution of animals’ shape, size and coloration.
Let’s return to the giraffe. Giraffes need to recognize and mate with others of their own kind—and not, say, with antelopes or okapi. Wired into the animals’ visual centers is a recognition system that automatically prefers mates that have more “giraffelike” characteristics. In this formulation, the longer necks were selected not because of any functional reason but simply because in scanning for desired traits, the visual system lights upon exaggerated ones first. They stand out, like Nixon’s prominent brows. Across successive generations, the long neck would have become an ever more reliable species marker for giraffeness, thereby enabling a partner to be spotted even from a great distance.
Our theory is not intended to replace Charles Darwin’s but to point out that other powerful forces besides the natural selection of fitness-conferring genes may be involved. Darwin, of course, acknowledged as much when he observed that mating behavior—so-called sexual selection—can exert its own, often maladaptive, impact on evolution. Because female peacocks prefer males with large tails, big-tail genes multiply in the population, eventually culminating in modern peacocks’ magnificent but absurdly impractical tails.