Environmental sensitivity describes the ability of an individual to perceive and process information about their environment. It is a basic trait found in many organisms that enables an individual to adapt to different environmental conditions. Levels of environmental sensitivity often vary considerably from individual to individual, with some being more and others less sensitive to the same conditions. Such differences have been observed across many species such as pumpkinseed fish, zebra finches, mice, non-human primates and humans, indicating that there is a biological basis to differences in sensitivity.
Theoretical background
The concept of environmental sensitivity integrates multiple theories on how people respond to negative and positive experiences. These include the frameworks of diathesis-stress modelMonroe, S.M. and A.D. Simons, Diathesis-stress theories in the context of life stress research: implications for the depressive disorders. Psychol Bull, 1991. 110(3): p. 406-25. and vantage sensitivity,Pluess, M. and J. Belsky, Vantage sensitivity: Individual differences in response to positive experiences. Psychological Bulletin, 2013. 139(4): p. 901-16. as well as the three leading theories on more general sensitivity: differential susceptibility,Belsky, J., Variation in susceptibility to rearing influences: An evolutionary argument. Psychological Inquiry, 1997. 8: p. 182-186.Belsky, J. and M. Pluess, Beyond diathesis stress: differential susceptibility to environmental influences. Psychol Bull, 2009. 135(6): p. 885-908. biological sensitivity to context,Boyce, W.T. and B.J. Ellis, Biological sensitivity to context: I. An evolutionary-developmental theory of the origins and functions of stress reactivity. Dev Psychopathol, 2005. 17(2): p. 271-301 and sensory processing sensitivity.Aron, E.N. and A. Aron, Sensory-processing sensitivity and its relation to introversion and emotionality. J Pers Soc Psychol, 1997. 73(2): p. 345-68. These will be described briefly in turn, before presenting the integrative theory of Environmental Sensitivity in more detail.
Diathesis–stress
According to the long-standing diathesis–stress model, people differ in their response to adverse experiences and environments, with some individuals more negatively affected by environmental stressors than others. The model proposes that such differences in response result from the presence of "vulnerability" factors, which include psychological (e.g., impulsive personality), physiological (e.g., high physiological reactivity) and genetic factors (e.g., genetic variation in the serotonin transporter [5-HTTLPR short allele]). In other words, the model suggests that individuals carrying certain vulnerability factors are at greater risk of developing problems when exposed to adverse environments. While the diathesis–stress model is helpful in understanding differences in response to negative stressors, it does not consider or describe differences in response to positive experiences.
Vantage sensitivity
The vantage sensitivity framework was developed in 2013 by Michael Pluess and Jay Belsky to describe individual differences in response to positive experiences and environments. While some people appear to benefit particularly strongly from positive experiences (e.g., positive parenting, supportive relationships, psychological interventions), others appear to benefit less. Although a relatively new concept, a growing number of studies provides evidence to support the framework. While the Vantage Sensitivity framework considers individual differences in response to positive experiences, it does not make predictions about the response to negative experiences.
Differential susceptibility
Differential susceptibility, proposed by Jay Belsky, brings the differential responses to both positive and negative experiences together in one single model. Grounded in evolutionary theory, Belsky and colleagues sought to understand why and how children differ so fundamentally in their developmental response to external influences, with some being more and others less susceptible. Importantly, the theory finds that more susceptible individuals are not only more negatively affected by adverse experiences (as described in the diathesis–stress model) but also particularly positively affected by the presence of favourable conditions (as described in the Vantage Sensitivity model). According to empirical studies, Differential Susceptibility is associated with various genetic, physiological and psychological factors, some of which are described below (see Empirical Evidence). Although early research suggested that differences in individuals' susceptibility are rooted in genetic factors, more recent research has shown that susceptibility is also influenced by prenatal and early postnatal environmental factors.
Biological sensitivity to context The theory of biological sensitivity to context by Tom Boyce and Bruce Ellis is based on evolutionary thinking and proposes that an individual's sensitivity to the environment is shaped by the quality of early life experiences. For example, particularly negative or especially positive childhood environments are understood to predict greater physiological reactivity later in life. In contrast, sensitivity is expected to be lowest for individuals with childhood environments that were neither extremely beneficial nor extremely adverse.
Sensory processing sensitivity Sensory processing sensitivity (SPS) theory by Elaine N. Aron and Arthur Aron proposes that sensitivity is a stable human trait characterized by greater awareness of sensory stimulation, behavioural inhibition, deeper cognitive processing of environmental stimuli, and higher emotional and physiological reactivity. According to this theory, approximately 20% of people fall into the category of a Highly Sensitive Person (HSP), in contrast to the remaining 80% who are considered less sensitive. Furthermore, the theory suggests that the sensitivity trait is adaptive from an evolutionary perspective, and regards differences in sensitivity as genetically determined and expressed via a more sensitive central nervous system.
Environmental sensitivity The broad theory of environmental sensitivity integrates all of the listed frameworks and proposes that although all people are sensitive to their environment, some individuals tend to be more sensitive than others. Furthermore, the theory of environmental sensitivity suggests that people vary in their sensitivity to the environment due to differences in their ability to perceive and process information about the environment. In other words, more sensitive individuals are characterised by heightened perception as well as deeper processing of external information due to neurobiological differences in the central nervous system, which are influenced by genetic as well as environmental factors. The integrative perspective of environmental sensitivity further proposes that whilst some individuals are more sensitive to both negative and positive experiences, others may be particularly vulnerable to adverse experiences (but not very sensitive to positive ones), whilst some may be especially responsive to positive exposure (but not vulnerable to negative ones).
Empirical evidence for individual differences Differences in sensitivity have been studied in relation to a wide range of sensitivity markers, such as genetic, physiological and psychological ones.
Genetic markers A growing number of studies provide empirical evidence to support individual differences in sensitivity at the genetic level. These include traditional gene–environment interaction studies featuring candidate genes,Bakermans-Kranenburg, M.J. and M.H. van IJzendoorn, Differential susceptibility to rearing environment depending on dopamine-related genes: new evidence and a meta-analysis. Development and Psychopathology, 2011. 23(1): p. 39-52.Schlomer, G.L., et al., Extending Previous cGxI Findings on 5-HTTLPR's Moderation of Intervention Effects on Adolescent Substance Misuse Initiation. Child Dev, 2017. 88(6): p. 2001-2012. as well as more recent genome-wide approaches.Keers, R., et al., A Genome-Wide Test of the Differential Susceptibility Hypothesis Reveals a Genetic Predictor of Differential Response to Psychological Treatments for Child Anxiety Disorders. Psychotherapy and psychosomatics, 2016. 85(3): p. 146-158. As an example for the latter, Keers et al.Keers, R., et al., A genome-wide test of the differential susceptibility hypothesis reveals a genetic predictor of differential response to psychological treatments for child anxiety disorders. Psychotherapy and Psychosomatics, 2016. created a polygenic score for environmental sensitivity based on about 25,000 gene variants across the genome and then tested whether children at the extremes of this spectrum of genetic sensitivity differed in their response to the parenting quality they received. According to the results, children with high genetic sensitivity were more likely to develop emotional problems when experiencing negative parenting but also less likely to develop problems when parenting was positive. On the other hand, children with low genetic sensitivity were not as impacted by the experience of negative or positive parenting, and did not differ from each other in emotional problems based on parenting quality. Hence, this study provides important evidence that genetic sensitivity, measured across the genome, predicts children's sensitivity to both negative and positive environmental influences.
Physiological markers Several empirical studies have reported differences in sensitivity to the environment in relation to individuals' physiological reactivity. Higher reactivity generally appears to reflect greater sensitivity. For example, research has shown that children with a higher physiological response to stress (indicated by the hormone cortisol) are more strongly affected by their family's financial situation.Obradovic, J., X.A. Portilla, and P.J. Ballard, Biological Sensitivity to Family Income: Differential Effects on Early Executive Functioning. Child Dev, 2016. 87(2): p. 374-84. More specifically, a high cortisol response in children was associated with more positive cognitive development when family income was high, but with reduced cognitive development when family income was low. In contrast, family income was less relevant for the cognitive development of children who displayed low physiological reactivity (i.e., a low cortisol response). Similarly, adolescents with high cortisol levels have been found to report more stress when experiencing school-related challenges but also the lowest stress in less demanding situations, whereas adolescents with lower cortisol levels were generally less affected by either low or high school-related challenges Xu, Y., et al., Interaction Effects of Life Events and Hair Cortisol on Perceived Stress, Anxiety, and Depressive Symptoms Among Chinese Adolescents: Testing the Differential Susceptibility and Diathesis-Stress Models. Front Psychol, 2019. 10(297): p. 297.
Psychological markers