Two speculations without time to investigate them at the moment.
We are portrayed as being in the midst of an "obesity epidemic." Regardless of the merits of that assertion, I think it is reasonable to conclude that people are indeed heavier today than they were a century ago. You look at paintings and photographs from 1900 and people do look much leaner. And they commonly are smoking.
Hence my question. Is the apparent obesity epidemic really two intersecting lines creating an amplified affect? The world became more prosperous and food became cheaper which would ceteris paribus lead to greater consumption and hypothetically greater weight. But what if the real issue was not inability to consume (because the causal structures of weight gain and loss are complex and little understood) but rather reduced to desire to consume because people smoked (an appetite suppressant)? Is obesity increase an unintended consequence of smoking decrease?
Second question - Yawning is an oddly sociable event. When one person in a room yawns, odds are high that someone else will do so as well within a minute. The act of yawning can cause a cascade of yawns or a chain reaction of yawns. It is a well established phenomena that is not well explained. Commonly posited causes include oxygen deprivation and empathy. Chimpanzees are also known to exhibit chain reaction yawning but I don't know about other animals. Yawning, certainly, but not necessarily the yawning cascade of humans.
My question is why the effect does not work cross species? We have multiple dogs and cats. They all yawn. Their yawning has no impact, it appears to me, on the human propensity to sympathetically yawn. Why?