By contrast technological change is very logical, as innovations provide greater efficiency and conveniences at lower costs. Our ancestors adopted the spinning wheel not as a “fad,” but because it shortened the time required to twist fibers into yarn. From this perspective cultural change appears bizarre. What were Stu and his imitators hoping to accomplish with a moptop? What changed their taste? Neither evolutionary biology nor economics can explain this behavior—the moptop has no intrinsic value over other styles, nor offers more tactile pleasure. Was the moptop a form of self-expression? If so, how did everyone know what feeling this particular haircut expressed? And why would everyone seek to express the same emotions through the same haircut at the same time?

But cultural changes are never random, nor do they befall us as plagues. Trends happen because individuals choose to take up new behaviors. And when we examine the history of cultural change, there are clear patterns in how humans move from one practice to another. Sixty years before the moptop, social scientist William Graham Sumner seemingly predicted how it would rise and fall: “A new fashion of dress seems at first to be absurd, ungraceful, or indecent. After a time this first impression of it is so dulled that all conform to the fashion.” In almost all instances, new behaviors begin as an exclusive practice of smaller social groups—whether elites or outsiders—and then eventually spread to the wider population. This is true for the diffusion of superficial hairstyles but also applies to things not considered “fashions”: practical technologies like cars and hybrid seed corn, delicacies like chocolate and gin, political and spiritual beliefs, and the succession of artistic movements in modern art. The thing we call culture is always an aggregation of individual human behaviors, and if taste were the mere product of random idiosyncrasies and irrational psychologies, culture would display no patterns, only noise. The fact that preferences in these disparate fields follow a similar rhythm of change suggests there must be universal principles of human behavior at work—the presence of a “cultural gravity” nudging humans into the same collective behaviors at the same time.