Some people buy the same perfume for decades. The same bottle, the same scent, never trying anything else. The industry calls that a lost customer. Psychology says those people know something the advertising would rather not say.

Smell is the only sense that goes straight to the part of the brain that stores emotions and memories, without routing through logic. That is why perfume works differently from everything else we put on ourselves.

Research by Rachel Herz and Jonathan Schooler showed that a memory recalled in the presence of a scent produces a more intense emotional experience and a stronger sense of being returned to the original moment - stronger than when the same memory is triggered by an image or a word. Another study with 170 participants, published in the journal Memory, found that scents linked to childhood evoke richer autobiographical memories than the corresponding photographs.

The psychologist Ana Galan explains it in a sentence worth rereading. If you use the same perfume for years, that scent accompanies you through experiences, relationships, journeys, losses and changes - and attaches itself to all of that history. In time, she says, you are no longer smelling a perfume: you are smelling part of your own biography.

That is why, when you catch the same scent on a different person years later, the reaction is uncontrollable. Galan describes it as a key that opens memories you were not looking for at all.

The psychologist Leticia Martin Enjuto adds the mechanism. The brain works through associations. If for years you have tied a particular scent to safety, intimacy or calm, minimal exposure to that scent reactivates the same emotions. Often, she says, you are reacting to the perfume and to everything it represents. Repeated use in the same context creates automatic associations, and the perfume becomes a psychological ritual bound to calm, concentration or confidence.

Hence the image of invisible armour. Wearing the same scent is a ritual that connects you to a particular version of yourself - the one you want to be that day.

All of this runs against the model the industry operates on. A new scent every season, a new story with every campaign, something you must try every month. If the truth is that a perfume is worth more with every year you have not changed it, then the best advice there is happens to be the costliest for the people selling it. Who has an interest in telling you that?