Compared to other animals, humans are uniquely able to synchronize our actions with each other. In fact, one provocative theory about the origin of human dance and song is that they allowed our ancestors to look to predators like a very large animal. And there’s a plausible sociology account wherein synchronized behaviors are at the heart of human emotional energy.
Peasants through history were often in synch. Such as when they sowed or reaped the fields, did folk dancing, or marched or sang in frequent rituals. Or when they marched as soldiers or rowed war galley ships. Early schools also relied heavily on synchronized recitation. And factory workers synched to their machines.
Oddly, people today seem quite reluctant to synch. Common styles of dance have drifted over time toward more individual and pair synchronization, and drifted there first among elites. And we now tend to see local synch as cringe. For example, imagine that several people at a social event all wore the same outfit. Or that several people at a restaurant table all ordered exactly the same food and drinks. Or imagine several people walking exactly in synch abreast down a busy sidewalk. These all seem cringe, unless they can be clearly seen as silly fun or some sort of artistic performance. Why?
Back when peasants and other lower class workers were highly synchronized, their more elite superiors were less synchronized. And thus synchronization at work may have become a sign of low class, a sign that at least some of the synchronized people are being dominated by and submitting to others. And so as we have become rich, and all see ourselves as higher class, we have all plausible become more averse to visibly synchronizing, unless it can be in a clearly leisure (like stadium waves) or high class (like dance troupe) context.
I count this as more evidence in favor of my theory that fertility is falling because we all see ourselves as high class, and are applying ancient heuristics that high class folks should have lower fertility. (Most ancient elites limited their fertility via infanticide or infant exposure.)
Also the songs of sailors - who coordinated work and kept spirits up by singing shanties.
Where do you get that ancient elites had lower fertility. Polygamy is specifically so that elites could have more babies. And in fact, elite women had better access to wetnurses, so they could return to fertilty sooner. And had better nutrition. Low fertility being an elite thing is a modern notion, I think. You're right about infanticide being a way to reduce number of babies.