Love at First Sight, or Love at First Smell?

Subjective experiences are internal perceptions that cannot be directly understood by others, such as happiness, sadness, pain, and jealousy. Determining the origin of these first-person experiences is challenging. Love is a subjective feeling that can strike you unexpectedly when you meet someone, or it can develop gradually over time. Once you recognize that emotion inside you, you decide whether to pursue a relationship with that person. We might think this romantic feeling comes from their appearance, but sometimes it is sparked by something deeper, qualities you cannot even see right away. We cannot fully understand how some emotions, thoughts, desires, and subsequent decisions emerge in the brain, although we may rationalize them. These ancestral and evolutionary mechanisms help us unconsciously behave in ways that promote survival and reproduction.

Pheromones and Chemical Communication

Pheromones are chemical signals that influence members of a species possessing this ability. While hormones regulate the responses of tissues and organs under various conditions, pheromones influence other individuals within a species, stimulating them to behave in specific ways. This is a form of chemical communication among individuals in certain animal species. In some cases, it helps establish territorial boundaries for animals like lions, tigers, and dogs. Additionally, in other species, including insects, nematodes, and some animals, pheromones act as signals to attract mates and facilitate reproduction. These chemicals, which can be volatile or non-volatile, enable one individual to identify a potential mate of the correct species and reproductive state by detecting signals from another.

Although there is not enough evidence for the existence of pheromones in humans, some observations suggest that similar mechanisms may influence those nearby, such as the synchronization of menstrual cycles among girls and women living together. These mechanisms may unconsciously influence our desires and decisions, especially when seeking a partner.

Major Histocompatibility Complex, Body Odor, and Human Attraction

The major histocompatibility complex (MHC), known as human leukocyte antigens (HLA), is located on the membrane of all cells in the body. It functions like a showcase, presenting internal antigens or foreign antigens, such as fragments of proteins from infected pathogens. The immune system cells, such as T-cells, are instructed to recognize their own antigens presented by MHCs; however, they act on cells exhibiting foreign antigens. MHC genes are a large set of genes found in most vertebrates. Humans have at least 224 MHC or HLA genes located on the short arm of chromosome six. MHC genes are the most variable genes in the human genome, resulting in no two individuals having identical HLA proteins, except for identical twins. The set of MHC genes we inherit makes each of us unique compared to other humans and distinguishes our own bodies from others.

There is evidence that MHC phenotypes are linked to body odor due to their influence on the types of small peptides they bind. It appears that the specific peptides bound to MHC affect the composition of compounds in sweat or the microbial communities on the skin surface, thereby influencing the smell of sweat. The microbes on our skin break down secretions from our sweat glands. The resulting body odor is heavily influenced by our MHC genotype.¹

Interestingly, Wedekind et al. (1995) demonstrated that women prefer the body smell of men with a more diverse MHC genotype. This preference seems to operate largely on an unconscious level. Women typically cannot explain why a certain odor is more attractive to them; however, they tend to rate it as more pleasant when it comes from men who are MHC-dissimilar. It is not like there's one specific chemical that acts as a universal love potion. Instead, it's a subtle mix of many different smells that our brains naturally respond to. The crucial takeaway is this: We are instinctively drawn to the natural scent of people whose immune systems differ from our own.²

When two parents have more different MHC profiles, their offspring will inherit a broader, more diverse set of MHC genes. This gives them a much wider "arsenal" to recognize and fight off a vast array of pathogens, making them healthier and more likely to survive.

The Immune System's Effect on Mental States

The immune system, in addition to its role in recognizing and removing pathogens and damaged cells, has a remarkable effect on mental states, such as emotions, thoughts, and desires, through unconscious mechanisms. These effects further highlight and confirm the interconnected roles of bodily systems in cognition and the mind, and vice versa. This field, known as psychoneuroimmunology, is a new and progressive discipline that not only challenges old beliefs about the mind but also provides new opportunities to treat both mental and physical diseases.

Falling in love with a certain person is influenced by several factors, including socioeconomic, psychological, and biological elements. Among socioeconomic factors, closeness to others, social class, cultural similarity, and shared beliefs are significant determinants. Psychological factors that affect partner choice include previous experiences and memories, family upbringing, and the need to love and be loved. Hormones and neurotransmitters such as dopamine, oxytocin, vasopressin, and serotonin play crucial roles in pleasure, intimacy, loyalty, and mood, thereby influencing the initiation and continuation of relationships. However, some bodily mechanisms may unconsciously determine our decisions to choose someone for a romantic relationship. MHC profile, which is expressed on the membranes of all our cells, may drive us to choose someone with a different MHC phenotype to produce children with a more diverse set of MHC genes. This diversity results in offspring equipped with stronger immune systems, enabling them to better fight infections and diseases.