Carl Jung, the Swiss psychiatrist and psychoanalyst, was the first person to talk about “innate sensitivity.” Jung believed that around 25 percent of all people are born with a particularly sensitive disposition and that this sensitivity has a decisive influence on people’s worldview. Jung introduced the terms introversion and extroversion into personality psychology to describe two different natures that influence people’s perception, intuition, thinking, feelings, and behavior. According to Jung, introverted people are more inclined to direct their energy and their attention inward and toward their inner processes (feeling and thinking, for instance), whereas extroverted people are more strongly inclined to direct their physical energy outward.3 Since then, numerous researchers into personality traits, including Jung himself, have continued to develop the concept of introversion and extroversion.
One of these researchers was the German-born British psychologist Hans Jürgen Eysenck, who related Jung’s concept to Hippocrates’s temperamental theory and believed that there is a neurological basis for the differences between introverted and extroverted people. In 1968, he described the typical introvert as someone who is quiet, introspective, rather reserved (except with very close friends), and loves books more than people. Introverts tend to make plans in advance, be cautious, and not like impulsive actions. They don’t like arousal, approach daily life with a certain seriousness, and value a well-ordered life. Eysenck describes the typical extrovert as sociable, as someone who likes events, has many friends, needs people to talk to, and doesn’t like being alone. Extroverts crave excitement, are constantly making the most of opportunities, react spontaneously, take risks, and are generally more impulsive.4
As such, Jung’s concept of innate sensitivity began to shift to a difference between observable extroverted and introverted behaviors in people. Jung’s theory of extroversion and introversion continues to be hugely important, and it has had a decisive influence on research into both temperament and personality. In the most commonly used model of personality psychology, the Big Five, extroversion is included alongside openness to experience, conscientiousness, agreeableness, and neuroticism. And there continues to be widespread interest in the concept of extroversion and introversion outside of academic research, as evidenced by the success of books like Susan Cain’s brilliant bestseller, Quiet: The Power of Introverts in a World That Can’t Stop Talking.
Another researcher influenced by Jung’s theory of extroversion and introversion is Jerome Kagan, professor of developmental psychology at Harvard University. Based on the results of his longitudinal studies, begun in the 1970s, Kagan differentiates between two groups of children: inhibited children and uninhibited children. According to Kagan, these two types represent relatively stable temperamental traits that follow us throughout our lives and that can only be influenced by environmental factors to a limited extent. Schneider summarizes Kagan’s results as follows:
Behavioral inhibition can be defined as a withdrawn, cautious, avoidant, and shy behavior in new and unfamiliar situations, such as meeting new people or dealing with unfamiliar objects and environments. This behavior can already be evident at the age of eight months. In babies, behavioral inhibition manifests itself as an easily triggered irritability (for instance, crying or screaming), in infants as shy and anxious behavior, and in school children as socially withdrawn behavior. The stability of this temperamental trait into adolescence has been demonstrated in a number of studies.5
According to Kagan, around 20 percent of all children exhibit inhibited behavior. These children have a lower arousal threshold than other children, particularly in unfamiliar situations. This means that their sympathetic nervous systems respond in a more reactive way to these stimuli. The sympathetic nervous system is part of the autonomic nervous system, alongside the parasympathetic nervous system, which is involved in activities such as digestion when we are at rest. The sympathetic nervous system, on the other hand, is involved in stimulating activities that affect our heart rate, blood pressure, muscle tone, and metabolism. When confronted with an unfamiliar situation or a new stimulus, inhibited children—in contrast to uninhibited children—will exhibit shy, cautious, and withdrawn behavior, while simultaneously exhibiting increased stress symptoms in their sympathetic nervous system, such as muscle tension and a heightened heart rate.
Numerous other researchers on temperament, including the psychiatrists Alexander Thomas and Stella Chess, have developed a range of different categories and models to differentiate between various temperamental traits. In their longitudinal study on temperamental development, which ran from 1956 to the 1990s in New York, Thomas and Chess observed the behavioral characteristics of babies and defined nine new temperamental dimensions.6 They were able to assign a clear temperamental type to 65 percent of the babies: 40 percent were categorized as “easy” babies, 10 percent were “difficult” babies, and around 15 percent were categorized as “slow to warm up.” In a book on high sensitivity, you can probably guess that it is the babies who were “slow to warm up” that we are interested in. The babies in this group were withdrawn when they had to deal with new people or situations and needed more time to get used to them. This means that they were initially behaviorally inhibited, but they then particularly benefitted from repeated contact and increased familiarity with new situations, people, or objects.7, 8 Their activity levels were lower and their sensitivity to subtle stimuli greater, and they reacted less emotionally than babies with “difficult” temperaments.
What these scientific findings suggest is that Jung was probably right when he posited that “many people are more sensitive than others from birth onwards.” And it also seems to be the case that children described as “inhibited” have similar characteristics to those described in other studies as “slow to warm up”: a stronger physical and emotional reaction to new and unfamiliar situations and stimuli and withdrawn behavior. What research has also been able to show is that alongside visible differences in behavior, there are also underlying physical and biochemical differences between inhibited and uninhibited children, as well as between extroverted and introverted adults. Introverted people, for instance, display a lower pain threshold and generally react more sensitively to external stimulation, such as visual and aural stimuli.9 Both the British psychologist Jeffrey Alan Gray and the American psychiatrist C. Robert Cloninger have created influential models that suggest that personality differences between people can be explained by biological causes.10
For a long time, though, inhibited behavior among children had been judged negatively because it was connected with the development of anxiety disorders in adulthood. A sensitivity to new environmental stimuli was seen as representing a higher level of vulnerability or susceptibility and was thus judged to be a risk factor in the development of psychological problems, as well as being connected with shyness in both children and adults.
But could being more sensitive to external stimuli actually be advantageous? And could the same fundamental higher sensitivity be the underlying cause of all of these different behavioral characteristics, be they “slow to warm up,” “behaviorally inhibited,” “withdrawn,” or “introverted”? It was these questions that a series of researchers began to ask in the 1990s, with fascinating results.
The Advantages of Being More Sensitive to Your Environment
So we have now learned that research into temperament suggests that, pretty much from birth onward, people register information from their environment differently from each other and also that they differ in their observable reactions and behaviors. These differences in sensitivity are not only seen among people, but have also been observed to date in over one hundred different animals, including rhesus monkeys, mice, dogs, zebra finches, fruit flies, and fish.11
Be it in a human being or a zebra finch, we can observe two distinct strategies when animals or people are faced with new or what initially appear to be threatening situations. One group behaves reactively, that is to say, they wait and become observant and cautious before they act. The other