Guest Article

While still considered a relatively new concept within the field of psychology, the trait of sensitivity has been actively researched by academics and practitioners for at least 25 years. Hence, it is the right time to take stock of what we have learnt to-date and what we need to investigate in future studies. A better understanding of why and how some people are more (and others less) sensitive is important because it will inform us about the different abilities and needs of people with different levels of sensitivity. The roots of sensitivity research actually stretch back 100 years to the early days of psychoanalysis when psychiatrist C.G. Jung proposed that some people are characterised by “an innate sensitiveness” (1). Since then, some aspects of sensitivity have been investigated under different terminology (e.g., introversion or behaviour inhibition) in separate lines of psychological research but it wasn’t until the mid-1990s when more specific theories on sensitivity emerged and researchers began to investigate sensitivity as a trait in its own right. These new theories sparked broad interest and stimulated new research. In what follows, I will attempt to summarise and describe this more recent research from the past (1995-2015) and the present (2015-2020), and identify directions for research in the future (2020-2030). Given the large number of studies undertaken over this time period (2), the current summary does not seek to cover all of the various contributions to the field but rather highlights those that are most relevant for the purpose of this short article.

Past: Description of Sensitivity and First Empirical Evidence (1995-2015)

The first 20 years of sensitivity research largely focused on the development of psychological theory. Indeed, it is critical to have a solid theory in place before conducting empirical research to test and advance it. Three individual theories from different researchers emerged around the same time as a response to clinical observation or academic research on child development. The theories were Sensory Processing Sensitivity (SPS) by Elaine and Art Aron, Differential Susceptibility (DS) by Jay Belsky, and Biological Sensitivity to Context (BSC) by Tom Boyce and Bruce Ellis. The common thread shared by these theories is that they all suggest that some people are especially strongly affected by what they experience. During the early days of sensitivity research, studies focused narrowly on the specific aspects of each of these theoretical perspective, such as the mechanisms thought to underlie sensitivity. For example, SPS research focused primarily on personality in adults, DS research focused on infant temperament, and BSC research focused on physiological stress reactivity in children. An important early contribution was the development of a self-report measure of sensitivity for adults known as the Highly Sensitive Person (HSP) scale. This paved the way for a large number of follow-up studies that looked at how sensitivity was related to other traits such as introversion. While empirical research related to SPS primarily involved cross-sectional study designs and adult samples, research on DS and BSC predominantly featured longitudinal studies covering child samples from early childhood through to adolescence. During this period, a growing number of studies provided increasingly stronger empirical evidence for the concept of sensitivity with first studies also exploring brain function and genetics of sensitivity.

Present: Refinement of Theory and Expansion of Research (2015-2020)

The last five years of sensitivity research have been shaped by the refinement and consolidation of the theories underpinning the construct, and the broadening and expansion of empirical research allowing for a deeper understanding of the psychological, physiological, and genetic components of sensitivity. The various theories and constructs were combined into a broader integrated framework of Environmental Sensitivity (ES) by Michael Pluess (3,4). New ways of measuring sensitivity in children and adolescents were developed, including assessments based on behavioural observation by trained experts. Up to this period, several theories tended to differentiate between two groups of people: those who are highly sensitive and those who are not. However, new studies in much larger samples during this period led to the discovery that sensitivity should be considered along a continuum (everyone is sensitive to an extent) and that people can be categorised into three sensitivity groups: low, medium, or high. These groups were labelled as dandelions, tulips, and orchids. During this period, significant progress was also made regarding the relationship between sensitivity and other common personality traits, pointing to a specific personality profile underlying sensitivity. Specifically, research found that sensitivity is characterised by heightened neuroticism and openness to experiences, with introversion playing a smaller role than previously assumed. In relation to the neuroscience of sensitivity, the structure and function of several brain regions, such as the hippocampus and amygdala, were found to play an important role. Access to new measures and larger samples also allowed for substantial advancements in our understanding of the role of genetics in sensitivity, with studies finding that about 50% of the differences between individuals can be explained by genetic factors. Furthermore, these genetic factors are distributed widely across the whole genome rather than reflecting a single “sensitivity gene”. Empirical research continued to build and expand into geographic locations, cultures, and contexts beyond the USA and UK, such as Italy, Belgium, Germany, Lebanon, Japan, and South Africa, to mention just a few examples. Finally and importantly, research designs were also strengthened, over this time, with studies adopting more experimental and longitudinal approaches. They also increasingly investigated sensitivity in response to positive experiences rather than focusing on predominately negative ones, highlighting the many benefits of high sensitivity. 

Future: Measurement, Biology, and Development across the Life Course (2020-2030)

Although there has been significant progress in research on sensitivity over the last 20 years, our current knowledge has gaps that need to be addressed in future research. Among these is the question of how exactly sensitivity develops over time and whether it is set in childhood or can develop further in adulthood. In order to investigate this, we need to continue improving our ability to accurately measure sensitivity by identifying and capturing the most essential features of sensitivity. Ideally, such measures will be objective, applicable to people across different ages and cultures, and include biological components of sensitivity. While there has been some initial progress in our understanding of the biology underlying sensitivity, much more work is needed with a focus on neuroscience, physiology and genetics. Carefully planned neuroscientific and physiological studies are fundamental in order to advance our understanding of sensitivity. However, genetic studies may be more challenging to undertake given that they require very large sample sizes (>100,000 people). Finally, improved measurement of sensitivity is also vital to advance our understanding of the relationship between sensitivity and mental health. 

Conclusion

The seeds of early research on sensitivity, sown 25 years ago, have sprouted and grown into a solid tree. With an increasing number of colleagues across the globe joining research efforts, this tree is likely to grow substantially over the next 10 years. At the same time, sensitivity has also been gaining more attention in the public eye as evidenced by the increasing number of books, blogs, and media coverage on the topic. In short, these are exciting times for research on sensitivity! Whilst we have come a long way already, the journey continues and is likely full of exciting discoveries. For information and updates on the latest research as well as access to online sensitivity self-tests, keep visiting our website www.sensitivityresearch.com which is run by a group of researchers dedicated to sharing reliable knowledge on the human trait of sensitivity. 

References

1. Jung, C. G. (1913). The theory of psychoanalysis. Psychoanalytic Review, 1(1), 1-40.

2. Greven, C. U., Lionetti, F., Booth, C., Aron, E. N., Fox, E., Schendan, H. E., . . . Homberg, J. (2019). Sensory Processing Sensitivity in the context of Environmental Sensitivity: A critical review and development of research agenda. Neuroscience and Biobehavioral Reviews, 98, 287-305. doi:10.1016/j.neubiorev.2019.01.009

3. Pluess, M. (2015). Individual Differences in Environmental Sensitivity. Child Development Perspectives, 9(3), 138-143. doi:10.1111/cdep.12120

4. Pluess, M., Lionetti, F., Aron, E., & Aron, A. (2020). People Differ in their Sensitivity to the Environment: An Integrated Theory and Empirical Evidence. PsyArXiv

Michael Pluess is a Professor of Development Psychology and leading expert on sensitivity in children and adults. He has made significant theoretical and empirical contributions in the field, along with the development and validation of sensitivity measures. He leads several large research projects on sensitivity across the world.

Julie Bjelland is a Psychotherapist, host of The HSP Podcast, and Founder of the Sensitive Empowerment Community, whose mission is to create a paradigm shift where sensitivity is embraced, valued, and honored. Register for free Masterclasses, get a free guide to calm your brain, a letter to give your medical and mental health practitioners about high sensitivity, and take the Sensitivity Quiz at JulieBjelland.com ❤️🌈❤️ (she/her)