Essentials: How Smell, Taste & Pheromones Shape Behavior

This Essentials episode covers chemical sensing in three parts: smell, which detects odors in the environment; taste, which detects chemicals placed in the mouth and digestive tract; and chemicals made by other people that can affect how we feel, our hormones and our health. The host flags at the start that the last category, sometimes called pheromones, is contested in humans.

There actually hasn't been a clear example of a true human pheromonal effect.

The episode separates that open question from a claim it calls undeniable: humans make and release chemicals in tears, on the skin, in sweat and even in breath, and these chemicals can modulate the biology of other people. Chemicals reach us in different ways. Volatile particles are inhaled into the nose, as with the smell of a rose or cake. Foods are chewed and broken down so the tongue can sense them. Some intake is deliberate, such as choosing and chewing a food. Some is not, such as walking into a smoky room and reacting to the smell. A third route involves chemicals that other people actively produce, through breath, tears or, which the host stresses is only possible, pheromones that enter through the nose, eyes or mouth.

The main example is a study described as published about ten years ago in the journal Science. Women were recruited for a high tendency to cry at sad movies, a trait the episode notes not all women share. Their tears were collected, and male subjects smelled either these tears or a saline control. The researchers wanted tears produced by real sadness, not by an eye irritant. Men who smelled the sadness-evoked tears showed a significant drop in testosterone and reduced activity in brain areas linked to sexual arousal. The host does not present this as a finding about tears and hormones as such. It is used to show that chemicals made by other people can strongly modulate our internal state, a form of chemical interpersonal communication that the host says most people do not appreciate.

In the episode's account, smell starts with sniffing. Volatile chemicals cannot enter the nose unless they are inhaled. That is why smelling is harder with a blocked nose or while breathing out, and why people cover their nose when something smells bad. The nose's mucosal lining traps incoming particles. The olfactory bulb sits about two centimeters above the roof of the mouth. Its neurons extend out of the skull into that mucus and respond to different odorant compounds. They also send branches deeper into the brain, which split into three paths.