February 2011

The next time you are in the kitchen, try this experiment: pick up a box of butter (four sticks) in one hand and a box of saltines (four packets) in the other. Which is heavier? If you said the butter, you are not alone. Most people would identify the box of butter as the heavier object — even though, if you look at the labels, you’ll see that they both weigh exactly one pound! This is an example of the size-weight illusion, and it is incredibly common. Most people — very young children, people from different cultures, and even people who know ahead of time that the two object weigh the same — report that the smaller of two objects of equal mass just “feels” heavier. Why is this? Recent research suggests that the roots of this modern party trick (as well as our penchant for sports like baseball and football) can be traced back to the evolutionary origins of Homo sapiens and to the opportunities provided by a well thrown rock or spear.

Where's the evolution?

When we humans make judgments, we often feel that we are 100% in charge of our thinking — that we simply observe the world as it is and make a conscious decision about it — but in fact, our brains and sensory systems have built-in biases that may subtly (or not so subtly!) influence both our perceptions and decision-making. Psychologists have demonstrated lots of them: We pay more attention to observations that support our current ideas than to observations that contradict what we already think. If an object is moving to the right, we perceive it as being further to the right than it actually is. We think that sounds that get louder are changing more than sounds that get softer. We view travel routes with more turns as longer than straight routes, even when they are the same distance. And those are just a few of the biases that shape how we view the world.

Biases that are hard-wired — those that are not learned through experience with the world — have evolutionary explanations. Some are simply by-products of the evolution of another feature. For example, the bias in our perception of travel distance may be a side effect of how our brains have evolved to efficiently estimate the passage of time. Other biases are adaptations themselves. For example, as the source of a sound gets closer to us, the sound becomes louder. Perhaps our bias for overestimating the change in an intensifying sound is something of an early-alert warning system, sensitizing us to potential danger approaching, and was favored by natural selection. Those with this perceptual bias may have been more likely to escape attacks and survive to reproduce than those without the bias (though further studies would be necessary to determine if this bias is actually an adaptation).

Scientists have long noted the universality of the size-weight illusion and formulated different hypotheses to explain it: is it just an inconsequential side effect of how our brains estimate mass, or is there some hidden value to persistently underestimating the weight of large objects? Now new research suggests that this error may actually be an adaptation with roots in an important but sometimes overlooked aspect of human evolution: throwing skill. Picking up a ball and giving it a toss may be child’s play, but in fact, it relies on a complex set of skills and knowledge, as well as musculature and bone structure, that other primates don’t have. Chimpanzees, it turns out, have a bad arm and lousy aim. And while throwing skill may not seem that useful today (unless you’re a major league baseball pitcher, of course), to human ancestors living half a million years ago, it may have been invaluable. Throwing rocks and spears would have allowed ancient humans to bring down prey from a distance and take advantage of the element of surprise. Improved throwing ability could have given some early humans a big selective advantage over others in terms of their ability to get food.

How might the size-weight illusion aid throwing skill? When selecting an object to throw a long distance, size and weight are key. It is harder to throw a large, light object a long distance than it is to throw a smaller object of the same weight. (Just imagine throwing a beach ball and a ball bearing of equivalent weights. You’d be able to throw the ball bearing much further than the beach ball.) So, the larger the object, the heavier it must be in order to be thrown a long distance; however, the heavier the object, the more work is required to throw it a long distance. Balancing these two issues to select the ideal object for throwing is no simple task.

Scientists from the University of Wyoming and Indiana University decided to test whether the size-weight illusion might help people make judgments about the best objects for throwing. They had people judge the throwability of a set of objects of different sizes and weights and also asked them to judge which objects were of equivalent weights. Last month, they announced their results: objects judged to be equivalently throwable were also judged to be of equivalent weights! This suggests that our perception of an object’s mass in comparison to other objects could serve as stand-in for its throwability. In other words, if the researchers are right, when you are holding a box of butter and a box of saltines in your hands and perceive the butter to be heavier, you are really getting a grip on the throwability of those objects. They weigh the same amount, but your brain is unconsciously tuning into the fact that the butter would be a better object to throw.

Of course, you won’t be throwing cans of soup or boxes of butter at a passing wildebeest anytime soon. Human lifestyles have changed substantially since our basic anatomy and physiology evolved, but our bodies and brains still bear the marks of our deep evolutionary history. So the next time you find yourself impressed with a 60 yard pass for a touchdown or pick up a rock to skip across a lake, take a moment to reflect on the hundreds of thousands of years of evolution that have shaped what it means to act, feel, think, and throw like a human.