Mate selection has always been a central component of reproduction. Passing on one’s own genetic material and equipping offspring with a robust immune system are the primary goals. How does this selection process occur, and what does the sense of smell have to do with it?

From a biological perspective, humans’ primary function is to reproduce to perpetuate their species. Humans cannot do otherwise, because it’s in their genes. However, how two people ultimately find each other and fall in love remains a mystery.

Unconscious smelling

Some mammals, such as mice, find a suitable genetic partner through their sense of smell. This happens through the perception of so-called chemosensory scents. They are also called pheromones and are found in mammals and humans. While mice can actually smell pheromones with their Jacobson’s organ, this is not the case with humans. Humans also have this organ, but it is only rudimentarily developed. In humans, the process of pheromone perception takes place unconsciously. Even if it is not yet clear exactly how this happens, humans can use the messenger substance to determine whether someone is part of their family circle, whether someone is feeling fear or joy, and whether they like someone or not.

The more diverse, the better

A person’s pheromones are perceived as particularly pleasant when their immune system differs slightly from one’s own. The so-called MHC complex, a group of immune genes, plays a central role in this. The MHC genes are responsible for the defense against pathogens. With their help, the body builds molecules that enable the immune system to recognize and fight pathogens. Since there are a multitude of pathogens, as many different MHC variants as possible should be present. If this is the case, the person has a well-equipped immune defense system.

Cultural context

In a study by psychologist Prof. Ilona Croy of Dresden University Hospital, the immunogenetic data of 3,691 German married couples were examined and evaluated. Compared to other randomly selected couples, the genetic variability of the subjects’ MHC genes did not differ significantly. Prof. Croy and her team conclude that the choice of partner for a long-term relationship depends not only on immune genes. Cultural expectations, socioeconomic status, shared interests, and, depending on cultural tradition, family arrangements also play a significant role.

Smell has a particularly strong influence on sexuality. Prof. Croy also discovered this in a study: people who can “smell each other well” find their sexuality more pleasant than those for whom the opposite is true.

Pheromones

Pheromones are scents used by animals to communicate with each other. Their primary purpose is reproduction. Animals use pheromones to locate potential mates. Males use scent to detect when a female is fertile.

In humans, pheromone cells have their own access to the brain—pheromone receptors. Therefore, they are processed differently than normal odors. While we smell something when odor molecules encounter normal olfactory receptors, we don’t perceive any odor when the messenger molecules bind to pheromone receptors.