Supernormal Stimuli

by Deirdre Barrett

Evolutionary processes sometimes go too far. The standard example presented for this point is the peacock’s tail. There’s nothing inherently adaptive about a big showy tail. But long ago, when peacocks and peahens were fairly normal birds, a peacock with a slightly showier tail came along, and some chicks decided that this was a decent indicator of the peacock’s overall health. After all, if you’ve got any genetic defects, they’ll probably show up in the tail. It’s the same sort of snap assessment that people use: a guy with an expensive watch must be wealthy and powerful, right? So males purchase ever more expensive watches, not because they tell the time any better, but because they tell all around that this guy is probably rich and powerful.

Peacocks and peahens got caught up in that loop. Peahens who chose peacocks with showier tails did indeed do better than peahens who chose normal peacocks, so more and more peahens were bred with an inborn taste for showy tails. And of course males quickly selected for showy tails, and the process continued so far that modern male peacocks are burdened with a tail that’s so big and showy that 1) it attracts predators; and 2) it slows down the peacock and makes it harder for him to get around, catch food, find peahens, and so forth. This phenomenon is sometimes called “runaway evolution”.

There’s a behavioral analogue to runaway evolution, termed “supernormal stimuli”. This was first discovered in the 1930s by a Dutch scientist named Tinbergen who though it would be fun to play practical jokes on animals. In the most successful of these, the victim of the prank was a local bird that lays pale blue speckled eggs. While the bird was gone, Tinbergen would sneak up to the nest and add a fake egg that was a brighter blue, was slightly bigger, and had more pronounced speckles. The mother bird, upon returning, would treat the egg as if it were a really high-quality real egg. She’d sit on it in preference to her real eggs. Tinbergen continued to heighten the joke, adding bigger, bluer, and more strongly speckled eggs. Eventually he was using plaster eggs bigger than the mother, painted fluorescent blue, and painted with big polka dots. The mother would struggle to sit on top of this enormous egg. Why? Because her instincts tell her than an egg is anything that’s round, blue, and has speckles. So something that’s bigger, rounder, bluer, and more strongly speckled is simply a super-duper version of an egg – at least, to to the mind of a bird.

But don’t laugh at the poor bird too loudly – we humans do exactly the same thing. Consider human sexuality. Males like large breasts, narrow waists, and big butts, so we are treated to pornographic images of females with impossibly large breasts, ridiculously thin waists, and ballooning buttocks. Our standards of female beauty now range at the far end of the range of actual female bodily types. We idolize the women at the far end of the spectrum. Meanwhile, normal women make desperate efforts to enlarge their breasts while remaining thin.

Women have their flaws in this regard as well. They have always subconsciously known that just a few facial features communicate emotional states, and so to add to the power of their emotional appeal, they enhance those features. They paint their eyebrows darker, thicken their eyelashes, and increase the redness of their lips. To further enhance the conspicuity of their features, they paint their faces white. All this effort makes them feel more beautiful. It apparently works, but remember, they are super-sizing normality to achieve this.

Another good example of supernormal stimuli lies in the (primarily female) response to “cuteness” – faces that are made to look like infants. For example, compare these two teddy bears:

The first teddy bear is about a hundred years old; the second is modern. Notice how the teddy bear has evolved to be cuter by being more like a human infant. The snout is flatter, the limbs chubbier and shorter relative to the torso. The torso itself is shorter and round, just like a human baby. The nose is smaller but the mouth is wider. The instinctive neural circuitry in your brain seizes upon these visual features and interprets them as “cute” – so you buy the teddy bear.

Supernormal stimuli can really trip up people. The most obvious example is in our eating habits. In our hunter-gatherer days, fat and carbohydrates were especially rare and highly prized, so nowadays, when confronted by a cornucopia of fatty, sugary foods, we can’t help ourselves, and Americans grow fatter by the year. Talk about an evolutionary backfire!

Ms. Barrett goes over the edge when she discusses television’s appeal, which she considers to be another example of a supernormal stimulus. I won’t argue about that interpretation, but she certainly spends a lot of pages ranting about the evils of television. It’s rather embarrassing.

All in all, a good book marred by too much enthusiasm for her subject. Ms. Barrett is absolutely right in her overall theme that many of our instinctive behaviors hamper our success in modern civilization. I just wish she weren’t so preachy about it.