We all have that friend. Let's call her Jane. Jane is bubbly and gregarious. Jane doesn't attend a party, she manifests it. She seeks out social gatherings and is enlivened by human presence. Jane is a good listener and has many trusted friends. What can explain Jane's extroversion? The answer, astonishingly, is her attraction to human body odor.
Human body odor represents a ubiquitous and ancient social signal, linked to the domain of social expertise.
Indeed, human odor is continuously produced and carries a diversity of social and emotional information that is processed by the central nervous system without one's conscious knowledge. Previous research has uncovered the link between high social expertise (i.e. extroversion) and an exquisite sense of smell adept at identifying familiar persons by body odor. That is to say, Jane's social openness is directly related to her ability to use odor as a social stimulus. Following the scent of this theory, researchers in Germany posited that brain regions responsible for reward processing and social integration light up brighter in Jane-like individuals as compared to a timid counterpart.
Only girls allowed.
The study's participants were 26 women. Only women were used due to their greater smell-detecting ability as compared to men, although more importantly - for "their higher sensitivity to social cues." Prior to their brain scan, the women took a survey rating their level of agreement with phrases such as "It is quite easy for me to quickly get in a new group of people." Based on the answers, researchers classified the women into two groups: High level (HO) and low level (LO) of social openness. They did not include those in the middle range. The study, therefore, only used the most extremely extroverted and introverted individuals in their analysis.
Armpit sweat was collected in a cotton pad from both males and females donors during the course of one night. The females denied using hormonal contraception, and all donors were instructed to refrain from using deodorant and eating odorous foods such as garlic or onion. Pure, unused cotton pads were used as a control. While in the MRI scanner, the smell of all cotton pads (male, female and pure) was introduced bi-nasally through a tube to each of the participants in a random order. They were also asked to rate the odor's intensity (how perceivable they found it) and hedonic quality (how pleasing they found it).
So what does Jane's brain look like? Her right inferior frontal cortex (social perception) and right caudate nucleus (part of the reward system) are incredibly prominent. Social openness scores were found to correlate highly with activation in these regions. No surprise there, she finds social situations rewarding. The right inferior cortex is also responsible for empathy, which explains her good friendship skills. To boot, this same brain region is part of the mirror neuron system through which humans “mirror” the behavior of another. Jane’s caudate nucleus is also implicated in trust, reciprocal altruism and the anticipation of positive social encounters. Jane, in other words, is the quintessential social butterfly.
The nose knows.
The most interesting fact, however, is that the more intense and pleasant a human smell is - the more Jane's caudate nucleus lights up. That is to say, Jane associates human smell with a positive rewarding experience because it makes her happy. In fact, previous research has shown isolated individuals do not show activity in this reward system when encountered with human interaction, but do so with objects. This suggests an entirely different social reward pathway for individuals with low social openness.
Get wind of this.
As human odor is a significant and valued social signal, interesting questions arise. What about the interaction of odor with faces and/or voices? Would a more timid person’s reward pathway be sparked by a text instead of face-to-face interaction? What implications does social media have on an already socially isolated population? Is social openness a learned trait or genetically predisposed tendency?
These are all scent-illiating questions awaiting discovery.