Roughly one in five people, on the current best population estimates, notices the buzzing of a fluorescent light in an office that most other people cannot hear. Roughly one in five people finds a crowded restaurant substantially more depleting than the same restaurant would be at half the volume of noise. Roughly one in five people reports being deeply affected by the emotional tone of a room they have walked into, in a way that the other four people in the same room appear to have registered only superficially or not at all.
For most of the twentieth century, the standard clinical framing of these people was that they were more anxious, more neurotic, more introverted, or more emotionally fragile than the general population. The trait was treated as a subclinical weakness. It was something to be managed, coped with, or overcome. Popular psychology and clinical practice largely agreed that being highly reactive to sensory input was a problem to be reduced rather than a legitimate personality dimension to be understood.
That framing has, on the accumulated evidence of the past three decades, turned out to be substantially incomplete.
The trait
In 1997, two American psychologists, Elaine and Arthur Aron, published a paper in the Journal of Personality and Social Psychology in which they proposed that the tendency to process sensory and emotional information more deeply than average was a distinct personality trait rather than a symptom of anxiety or introversion. They called it Sensory Processing Sensitivity, or SPS, and they developed a twenty-seven-item questionnaire, called the Highly Sensitive Person Scale, to measure it.
The Arons’ proposal was, at the time, contested. The trait they were describing appeared to overlap substantially with neuroticism and introversion, and it was unclear whether SPS was genuinely distinct from those established constructs or whether the Arons were simply repackaging existing personality dimensions under a new name. The 1997 paper argued, on the basis of seven separate studies, that SPS was in fact partially independent of the two traits with which it had previously been conflated. The paper acknowledged that the argument required further empirical support.
Over the following two decades, that further support accumulated. Subsequent research, drawing on samples from multiple countries and using both the original HSP scale and refined versions of it, consistently identified SPS as a distinct three-component trait. The three components are called Ease of Excitation (the tendency to become overwhelmed by strong sensory input), Low Sensory Threshold (the tendency to notice subtle sensory information that others miss), and Aesthetic Sensitivity (the tendency to be deeply moved by art, music, and beauty). The three components correlate with each other but map on to different underlying neural and behavioural patterns.
Approximately 15 to 30 per cent of people, depending on the specific study and measurement method, score high enough on the HSP scale to be classified as highly sensitive.
The twin study
The strongest evidence that Sensory Processing Sensitivity is a genuinely heritable trait came in 2021, from a large behavioural genetics study using data from the Twins Early Development Study, one of the longest-running twin cohort studies in the world.
Elham Assary and colleagues at Queen Mary University of London and King’s College London, publishing in Molecular Psychiatry, analysed data from approximately 2,800 adolescent twin pairs, comparing scores on the HSP scale between identical and non-identical twin pairs. The comparison is the standard method for estimating the heritability of any human trait. Identical twins share close to 100 per cent of their genetic material. Non-identical twins share, on average, 50 per cent. The extent to which identical twins resemble each other more closely than non-identical twins on any given trait, controlling for shared environmental factors, provides an estimate of how much of the variation in that trait is attributable to genetic differences between individuals.
The Assary team’s estimate for Sensory Processing Sensitivity was approximately 47 per cent. Approximately half of the individual differences in how sensitive people are to their environment, on this analysis, are genetic in origin. The remaining half are attributable to environmental factors, including childhood experiences, learned behaviours, and interactions between genes and environment that the twin methodology cannot separately isolate.
The 47 per cent heritability figure is comparable to the heritability estimates for other well-established personality traits, including neuroticism, extraversion, and openness to experience. It places SPS on the same methodological footing as the traits it was previously suspected of merely duplicating.
The brain
The behavioural genetics finding was substantially strengthened by neuroimaging work published a decade earlier and refined since. Bianca Acevedo and colleagues at Stony Brook University, publishing in Brain and Behavior in 2014, used functional magnetic resonance imaging to compare how the brains of high-SPS and low-SPS individuals responded to images of familiar and unfamiliar faces displaying positive, negative, or neutral emotions.
The high-SPS participants, on the Acevedo team’s data, showed substantially greater activation in brain regions associated with awareness, integration of sensory information, empathy, and action planning. The specific regions included the anterior cingulate cortex, the insula, the inferior frontal gyrus, and the middle temporal gyrus. These are, on the standard neuroscience mapping, the brain regions that process both the emotional content of stimuli and the individual’s own physical and emotional state in response to those stimuli.
The interpretation was that Sensory Processing Sensitivity is not a matter of the brain being more easily overwhelmed by input. It is a matter of the brain performing substantially more processing on the input it receives, integrating that input with information about the self, and generating a more elaborate internal response than a low-SPS brain would produce from the same stimuli.
The evolutionary logic
The observation that Sensory Processing Sensitivity has been documented in more than 100 non-human species, from fruit flies to pumpkinseed sunfish to rhesus macaques, has substantially shifted the interpretation of what the trait might be for.
Populations of most social vertebrate species contain a minority of individuals who respond to environmental cues more intensely, notice subtle changes in their surroundings more quickly, and adjust their behaviour in response to new information more readily than the majority of their species. In evolutionary terms, these more sensitive individuals appear to be operating a different life-history strategy from their less sensitive conspecifics. The strategy involves higher up-front costs, including greater vulnerability to overstimulation and stress, in exchange
