It’s 1957 in the highlands of New Guinea. A woman’s husband has just died. She is grief stricken as any wife would be. She is now a widow. She calls on her brothers (or her next closest male kin). She insists that they must do what is proper in her culture. And it is not because the rest of the village forces her. It is because for generations she has seen all new widows take these ritual actions.
It’s 1279 in the great Song empire of China. Unfortunately, the empire is falling. It’s disgraceful end requires an act of valour by its leader. He must take his own life. He likely knows this and I would further argue that he would have wanted to do this rather than live in, what he may have deemed, disgrace.
The widow makes her closest male kin strangle her to death.
The emperor takes his own life. He is 8 years old. 800 members of his clan do the same with him.
It is the power of culture. It is the power that a grouping of humans can have over the wants, motivations and actions of its individuals no matter how little or great, or, positive or negative, those effects have from a evolutionary perspective or ability to further their genes in subsequent generations. What other people think is exceedingly powerful.