Human readiness to throw: the size–weight illusion is not an illusion when picking the best objects to throw
Long-distance throwing is uniquely human and enabled Homo sapiens to survive and even thrive during the ice ages. The precise motoric timing required relates throwing and speech abilities as dependent on the same uniquely human brain structures. Evidence from studies of brain evolution is consistent with this understanding of the evolution and success of H. sapiens. Recent theories of language development find readiness to develop language capabilities in perceptual biases that help generate ability to detect relevant higher order acoustic units that underlie speech. Might human throwing capabilities exhibit similar forms of readiness? Recently, human perception of optimal objects for long-distance throwing was found to exhibit a size–weight relation similar to the size–weight illusion; greater weights were picked for larger objects and were thrown the farthest. The size–weight illusion is: lift two objects of equal mass but different size, the larger is misperceived to be less heavy than the smaller. The illusion is reliable and robust. It persists when people know the masses are equal and handle objects properly. Children less than 2 years of age exhibit it. These findings suggest the illusion is intrinsic to humans. Here we show that perception of heaviness (including the illusion) and perception of optimal objects for throwing are equivalent. Thus, the illusion is functional, not a misperception: optimal objects for throwing are picked as having a particular heaviness. The best heaviness is learned while acquiring throwing skill. We suggest that the illusion is a perceptual bias that reflects readiness to acquire fully functional throwing ability. This unites human throwing and speaking abilities in development in a manner that is consistent with the evolutionary history.So take the kids out and skip stones, or throw snowballs. It's good for 'em.