The Highly Sensitive Children (HSC) and Sensory Processing Sensitivity (SPS)

It is notable that even newborns exhibit visible individual characteristics, and these observable individual differences are called temperament. For example, some infants cry easily and are hard to soothe once disturbed, while others sleep well despite noise. Traditionally, the first characteristics have been described as making up a 'difficult temperament' (Thomas & Chess, 1977), because they require more sensitive tender care than others. When they grow to express their preferences, they can be fussy about certain textures (e.g. clothing, food), prefer solitary play and show difficulty in social interactions, and at times show strong emotional reaction to seemingly minor occurrences. Research suggests that temperament has a neurobiological basis, concerning the mechanism whereby the child registers and responds to environmental information, and provides the foundation from which individual personality develops in later life (Rothbart, 2011).

Temperamental differences are also described in the field of personality research. Based on the study of highly sensitive adults (Highly Sensitive Person: HSP), Aron and Aron (1997) reported that an underlying Sensory Processing Sensitivity (SPS) contributes to the individual level of sensitivity, and high SPS is associated with a low threshold for environmental stimuli, strong emotional responsiveness, and deep information processing. Similarly, they describe children with high sensitivity as Highly Sensitive Children (HSC: Aron, 2002/Japanese translation by Akehashi, 2015), providing a useful conceptual foundation for parents who seek support in their parenting of 'difficult' children (Naganuma, 2017). In addition, it has been reported that HSC can be characterised as 'introverts' and 'withdrawn', which may cause difficulty in their social interactions, especially in collective contexts such as school settings.

On the other hand, that the issue of how SPS is distinct from other sensory specificity has been argued, particularly concerning sensory atypicality in cases of Autism Spectrum Disorder (ASD), which shares some characteristics of high sensitivity and hyper-responsiveness to environmental stimuli ^*1. In this regard, Acevedo et al. (2018) conducted a literature review of clinical fMRI studies of ASD and related disorders, and compared the suggested neural regions to those implicated in SPS studies. While they found common brain regions of activation in both clinical cases and cases of high SPS, they concluded that SPS differs in the response to reward processing of social stimuli, empathy, and the areas of self-control. However, as this review excluded studies of mild cases of ASD in order to draw a strong contrast with SPS, more empirical work will be needed to ascertain the factors distinguishing these seemingly related sensory features. Indeed, from a systematic literature review of SPS including psychological, biological, and psychiatric perspectives, Greven et al. (2019) proposed viewing SPS as a neutral cross-disorder (or transdiagnostic) trait ^*2 which is 'uniquely suited to bridge psychiatric disorders with biological substrates of behavior' (p. 301), suggesting its value in research to further advance our knowledge of clinical implications.

As mentioned above, highly sensitive characteristics in children can relate to difficulties in parenting, the children's social adjustment, and even their mental health conditions, which suggests the need for a supportive environment with preventative envisioning. Kushizaki (2018) also presented a conceptual review of SPS in relation to other psychosensory constructs (e.g. empathy, thin boundary, atypical sensory processing), and concluded by emphasising the need for awareness of the issues around HSC, including cases of school withdrawal.

Implications of Differential Susceptibility Theory

Thus far, the author has illustrated potential difficulties associated with high sensitivity in an attempt to urge the need to provide a supportive environment. That is, there is a certain proportion of individuals who are so highly sensitive to environmental stimuli that they are easily affected detrimentally. This view is particularly important to offer a preventative approach in the effects of person-environmental 'dual risk' interactions. Traditionally, this perspective has served as a useful framework, namely the diathesis stress model, which yielded valuable empirical findings regarding identifying the factors and mechanisms to prevent negative outcomes (e.g. Caspi et al., 2003).

Nonetheless, it is worth remembering that high sensitivity does not necessarily mean at risk, and what really matters is the Goodness of Fit (Chess & Thomas, 2013) of person-environments interaction. While valuing the scientific contribution of the diathesis stress paradigm, Belsky has reconsidered this dual risk model of person-environmental interaction and conceptualised individual sensitivity as a neutral susceptibility which can function both positively and negatively. This reconceptualization has led to the proposal of the Differential Susceptibility Theory ^*3 (DST: Belsky et al., 2007; Belsky & Pluess, 2009; Ellis et al., 2011; Kibe & Hirano, 2019), which posits that individual susceptibility functions as a vulnerability factor in a negative environment (i.e. a dual risk model), yet at the same time functions as a plasticity factor in a supportive environment. In short, within the DST framework, individual susceptibility can respond in a for better and for worse manner (Belsky et al., 2007), like the swing of a pendulum in accordance with a given context. This notion of differential susceptibility illuminates the neutral objectivity of individual sensitivity and sheds light on traditionally overlooked developmental plasticity. Fig. 1 presents both conceptual models; (a) depicts the diathesis stress model and (b) the differential susceptibility model, which incorporates positive aspects of development.

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Evidence from the DST framework indicates that what are traditionally considered vulnerability factors, such as a difficult temperament, high sensitivity, and some candidate genes relating to psychopathology, actually function as a plasticity factor in a positive environment. For example, emotionally dysregulated children showed significantly more positive responses than their counterparts when receiving high quality care and supportive intervention (Scott & O'Connor, 2012). Also, individuals with a 'risk gene', the serotonin transporter 5HTTLPR [ss, sl genotype] that has been reported as associated with psychopathology, presented much more favourable outcomes when they experience positive but not negative gene-environmental interactions (e.g. Van Ijzendoorn et al., 2012). Moreover, intervention studies using the above-mentioned SPS as an indicator of susceptibility revealed that high SPS moderates the level of externalising problems in kindergarten children (Slagt et al., 2018), changes of depressive symptoms in adolescent girls (Pluess & Boniwell, 2015), and the effects of psychological support in Japanese high school students (Kibe, 2018).

These empirical findings suggest that individual differences (e.g. genotype, temperament, personality) yield both positive and negative outcomes in response to the quality of environmental stimuli, and emphasise the importance of environmental accommodation according to such interpersonal differences. With regards to SPS, there are rhetorical descriptions of the 'orchid, tulip, and dandelion' in reference to high, middle, and low levels of sensitivity, respectively ^*4 (Fig. 2), which may allow intuitive conceptual guidance especially for those involved in childcare practices. Nevertheless, it would be feasible to view interpersonal differences in sensitivity along a spectrum, rather than as categorical entities; furthermore, it should be noted that intrapersonal variance would be observable in developmental phases as well. Lastly, it goes without saying that ensuring sensitive accommodation to meet each child's needs will allow the children to flourish regardless of their seed variety. In particular, as DST research in Japan is still in its infancy, further empirical findings will be needed to advance our knowledge of the effects of person-environment interactions.

• *1 Akehashi (2019) provides practical advice on this parenting topic.
• *2 Although Fairburn et al. (2003) proposed the Transdiagnostic Theory for the maintenance system of Eating Disorders, Greven et al. (2019) used this term to describe the functional aspect of SPS.
• *3 DST (Ellis et al., 2011) was proposed as a comprehensive neurodevelopmental theory founded on the Biological Sensitivity to Context Theory (Boyce & Ellice, 2005) and Differential Susceptibility Theory (Belsky et al., 2007). Furthermore, the conceptual approach to HSP/HSC based on SPS (Aron & Aron, 1997) has recently been discussed with regard to common contexts with DST as well (Ellice et al., 2011; Pluess et al., 2018; Kibe & Hirano, 2019).
• *4 Although the metaphor of 'orchid' and 'dandelion' has been used previously (Ellice et al., 2011), Lionetti et al. (2018) discovered the group of 'tulip' individuals in their empirical studies.

References

• Acevedo, B., Aron, E., Pospos, S., & Jessen, D. (2018). The functional highly sensitive brain: a review of the brain circuits underlying sensory processing sensitivity and seemingly related disorders. Philosophical Transactions of the Royal Society B: Biological Sciences, 373, 20170161.
• Akehashi, D. (2019). Oshiete Akehashi-sensei! Nanika hokano ko to chigau? HSC no sodatekata Q&A [Tell me, Dr. Akehashi! Is there something different with my child? Q&A on how to raise a Highly Sensitive Child (HSC)]. Ichimannendo Publishing Co. Ltd.
• Aron, E. (2002). The highly sensitive child: Helping our children thrive when the world overwhelms them. NY:Broadway Books. Translated by Daiji Akehashi as Hito ichibai binkanna ko. (Tokyo: Ichimannendo Publishing Co. Ltd., 2015).
• Aron, E. N., & Aron, A. (1997). Sensory-processing sensitivity and its relation to introversion and emotionality. Journal of Personality and Social Psychology, 73, 345-368.
• Belsky, J., Bakermans-Kranenburg, M. J., & Van IJzendoorn, M. H. (2007). For better and for worse: Differential susceptibility to environmental influences. Current Directions in Psychological Science, 16, 300-304.
• Belsky, J., & Pluess, M. (2009). Beyond diathesis stress: differential susceptibility to environmental influences. Psychological Bulletin, 135, 885-908.
• Boyce, W. T., & Ellis, B. J. (2005). Biological sensitivity to context: I. An evolutionary-developmental theory of the origins and functions of stress reactivity. Development and psychopathology, 17, 271-301.
• Caspi, A., Sugden, K., Moffitt, T. E., Taylor, A., Craig, I. W., Harrington, H., ... & Poulton, R. (2003). Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science, 301(5631), 386-389.
• Chess, S., & Thomas, A. (2013). Goodness of fit: Clinical applications, from infancy through adult life. New York: Routledge.
• Ellis, B. J., Boyce, W. T., Belsky, J., Bakermans-Kranenburg, M. J., & Van IJzendoorn, M. H. (2011). Differential susceptibility to the environment: An evolutionary-neurodevelopmental theory. Development and Psychopathology, 23(1), 7-28.
• Fairburn, C. G., Cooper, Z., & Shafran, R. (2003). Cognitive behaviour therapy for eating disorders: A "transdiagnostic" theory and treatment. Behaviour research and therapy, 41(5), 509-528.
• Greven, C. U., Lionetti, F., Booth, C., Aron, E., 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 & Biobehavioral Reviews. 98, 287-305.
• Kibe, C. (2018). Individual Sensitivity to the Effects of Resilience Education: Self-esteem Enhancement in Japanese Adolescents. Paper presented at the 7th Annual Conference of Applied Psychological Research in the Middle East.
• Kibe, C. & Hirano, M. (2019). Development of the Japanese Version of Highly Sensitive Child Scale for Adolescence （HSCS-A）. The Japanese Journal of Personality, https://doi.org/10.2132/personality.28.2.1 (in Japanese)
• Kushizaki, S. (2018). Understanding of "highly sensitive child": Autism, highly sensitive person, empath, and non-attendance at school Bulletin of the Institute of Human Rights Studies, Kansai University, 76: 27-55.
• Lionetti, F., Aron, A., Aron, E. N., Burns, G. L., Jagiellowicz, J., & Pluess, M. (2018). Dandelions, tulips and orchids: Evidence for the existence of low-sensitive, medium-sensitive and high-sensitive individuals. Translational Psychiatry, 8, 24-35.
• Naganuma, M. (2017). Kodomo no binkansa ni komattara yomu hon: jidouseishinkai ga oshieru HSC to no kakawarikata [A book to read when you are troubled: Pediatric psychiatrist's advice on how to interact with Highly Sensitive Children]. Seibundo Shinkosha.
• Pluess, M., & Boniwell, I. (2015). Sensory-processing sensitivity predicts treatment response to a school-based depression prevention program: Evidence of vantage sensitivity. Personality and Individual Differences, 82, 40-45.
• Pluess, M., Assary, E., Lionetti, F., Lester, K. J., Krapohl, E., Aron, E. N., & Aron, A. (2018). Environmental sensitivity in children: Development of the Highly Sensitive Child Scale and identification of sensitivity groups. Developmental Psychology, 54(1), 51-70.
• Rothbart, M. K. (2011). Becoming who we are: Temperament and personality in development. New York: Guilford Press.
• Scott, S., & O'connor, T. G. (2012). An experimental test of differential susceptibility to parenting among emotionally‐dysregulated children in a randomized controlled trial for oppositional behavior. Journal of Child Psychology and Psychiatry, 53, 1184-1193.
• Slagt, M., Dubas, J. S., van Aken, M. A., Ellis, B. J., & Deković, M. (2018). Sensory processing sensitivity as a marker of differential susceptibility to parenting. Developmental psychology, 54(3), 543-558.
• Thomas, A., & Chess, S. (1977). Temperament and development. New York: Brunner-Routledge.
• Van IJzendoorn, M. H., Belsky, J., & Bakermans-Kranenburg, M. J. (2012). Serotonin transporter genotype 5HTTLPR as a marker of differential susceptibility? A meta-analysis of child and adolescent gene-by-environment studies. Translational Psychiatry, 2, e147.