Environmental sensitivity
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 model and vantage sensitivity, as well as the three leading theories on more general sensitivity: differential susceptibility, biological sensitivity to context, and sensory processing sensitivity. 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, physiological and genetic factors. 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, 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
, 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 but also particularly positively affected by the presence of favourable conditions. According to empirical studies, Differential Susceptibility is associated with various genetic, physiological and psychological factors, some of which are described below. 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.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, 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, whilst some may be especially responsive to positive exposure.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, as well as more recent genome-wide approaches. As an example for the latter, Keers et al. 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 are more strongly affected by their family's financial situation. 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. 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 challengesPsychological markers
The majority of evidence for individual differences in sensitivity due to psychological markers of sensitivity is based on studies that investigate the interplay between infant temperament and parenting during childhood. Generally, higher fearfulness, fussiness, and negative emotionality in infancy have been associated with greater sensitivity to parenting quality. According to a large meta-analysis which summarises the findings from 84 individual studies, children that are characterised by a more sensitive temperament were more strongly affected by the parenting they receive. More specifically, sensitive children were more likely to develop problems when experiencing harsh and punitive parenting but also least likely to have problems when they were raised in emotionally warm and caring environments. Less sensitive children, on the other hand, did not differ much from each other whether they received more negative or more positive parenting.Determinants
An individual's level of environmental sensitivity is the result of a complex interplay between genes and environmental influences across development. Empirical studies suggest that genetic influences are important for the development of the trait, but only about 50% of the differences in sensitivity between people can be explained by genetic factors, with the remaining 50% being shaped by environmental influences. Furthermore, the genetic component of sensitivity is most likely made up by a large number of genetic variants across the whole genome, each making a small contribution, rather than a few specific genes. Importantly, although sensitivity has a substantial genetic basis, it may be the quality of the environment when growing up that shapes this genetic potential for sensitivity further. For example, those carrying a larger number of sensitivity genes may develop a sensitivity that is more geared towards threat when growing up in a challenging or adverse environment, whereas those growing up in a predominately supportive and secure context may develop heightened sensitivity towards positive aspects of the environment. Similarly, those that experienced similar levels of both negative and positive exposures during childhood may develop equal levels of sensitivity to negative and positive experiences.