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But even in the heartland of surströmming, it's hard to get a full commitment to it. We surströmming enthusiasts do our best to keep the family tradition alive; we comment and discuss exactly how much of each ingredient is needed—the chopped onion, the boiled potatoes, the tomatoes, the sour cream, the butter on the flatbread—and in what order they should be applied. We agree that the herring should be cleaned by hand, and that pre-filleted herring is somehow suspect, something that suits Stockholmers and other amateurs. It is more of a ritual than a dish. The smelly fish always leads to discussions, and they continue well into the northern summer evening. Only once is there silence, when my brother-in-law asks the not entirely innocent question: If the herring is as good as you say, why do you only eat it once a year? I answer with a quote from Ingemar Stenmark, the laconic downhill-skiing legend: "It's not worth explaining to those who don't understand." Then we change the subject.

Smelling is the easiest and most natural thing in the world. We do it all the time. We breathe in every four seconds. That's up to one thousand inhalations per hour and over twenty thousand times per day. The air we breathe in and out carries vital oxygen, which is sucked down our windpipes and into the alveoli, where it is transferred to the bloodstream and can oxygenate all parts of the body. But the air contains more than oxygen, and on its way to and from the lungs it passes through the nose, mouth and throat. There, a number of remarkable biological processes take place that shape how you experience the world. I and my colleagues at Stockholm University explore the vastness of the sense of smell—how smells create experiences, emotions, memories, behaviors. My research group consists of neuroscientists, psychologists, linguists, sensory scientists and computer scientists who share a common interest in the sense of smell. Nowadays, the human sense of smell is a popular research topic, but for many years it was almost completely unexplored. It was neglected and devalued by many philosophers and scientists. Moreover, until recently, odors were difficult to assess and describe with concepts and objective measuring instruments. A myth emerged that the human sense of smell is weak and insignificant. Therefore, the sense of smell still holds many mysteries waiting to be explored. This creates a pioneering spirit among us olfactory researchers.

The sense of smell is fascinating to anyone who has reflected on its elusive but powerful influence. Smells and scents are the world's best social icebreakers. When people hear what I'm researching, they immediately start talking. First there are discussions about food and wine, but pretty quickly they move on to personal topics: about the smells that evoke memories of childhood, dead relatives, pregnancy, love and sex. When people talk about their sense of smell, they also tell us something about themselves, their feelings, thoughts and relationships. Thus, the sense of smell not only reflects the chemical world outside us, but also bears witness to our inner world, our feelings and thoughts.

In the following chapters, we will explore the little-known depths of scent and how it shapes us: Why can smells evoke such strong memories? How much can body odors control who you fall in love with? How sensitive is the human sense of smell? Can smells make us work more efficiently, feel better—and keep the brain vital? How can the smell of our own children fill us with such strong emotions? Why can a sudden bout of nausea cause the smell of a favorite dish to become intolerable instead? And what are the nasal superpowers that allow wine tasters to pick out the grape varieties, regions and production processes of an ordinary wine? These are some of the questions that will be addressed.

My twenty-year research journey into the world of smell has been full of surprises, and with each question answered, several new questions have arisen. It is a journey that has broadened my view of what the sense of smell is. My insights shaped the content of the book you have in front of you. Three messages will permeate the book. The first is that the human sense of smell is extraordinarily sensitive. We have an incredibly well-developed ability to sense smells in our environment. So I want to dispel the myth that humans have a poor sense of smell. Fortunately, many people agree with me, and psychologists and brain researchers are now busy reassessing both human sensitivity to smell and the importance of the sense of smell in our lives.

The second message is that smelling is an intellectual process that starts in the brain. When I started my own research, I viewed the sense of smell as a simple, passive system: An olfactory impression starts with airborne molecules that travel through the nose and get stuck in the olfactory mucosa of the nasal cavity. This sets off a biological chain reaction that leads to signals in the brain, where our conscious olfactory experiences occur as reactions. This is the traditional view of the sense of smell. However, my research has increasingly led me to adopt a different perspective: olfactory processes start not in the nose but in the brain, even before the odor molecules reach their destination. Olfactory processes are shaped by expectations and experiences, which in turn carry with them life experiences and cultural conventions that we are often unaware of.

I call it the cognitive perspective. Cognition comes from the Latin cognoscere, which means "to learn," "to know" or "to recognize." Cognitive scientists have long wondered how we gain knowledge of the world through our senses, our thinking and our brains. In this book, I want to use the cognitive perspective to understand the sense of smell. I believe that the human sense of smell is smarter than you think. It has its own kind of intelligence.