These guys traveled the world in search of what makes things funny.
It doesn’t help that the term “humor” has had all sorts of different connotations. It wasn’t until the early nineteenth century that humor became widely used in its modern sense, as a virtue. Before that, “humour,” from the Latin word for “fluid,” referred to bile, phlegm, and other bodily fluids believed to wreak havoc on people’s moods. A “humourist” was someone whose body fluids were so imbalanced they acted mentally ill. A “man of humour” was someone skilled at impersonating an insane person.
Their working theory is illustrated as a Venn diagram. One circle represents something benign (“Grandpa”), the other circle is a violation of some sort (“erection”), the intersection is the funny (“Grandpa’s erection”).
As a side note, they made reference to a website I’m happy to report is absolutely real, Animals Being Dicks, which makes the authors my new besties.