We know a lot nowadays about how to extrapolate from rats to people, but we don't only have to rely on that. In a sense we've made ourselves into experimental animals. There are too many of us, too crowded, in an environment we've poisoned with our own-uh-byproducts. Now when this happens to a wild species, or to rats in a lab, the next generation turns out weaker and slower and more timid. This is a defense mechanism.