Highly sensitive people, hyperreactive amygdala and the hunter-gatherer hypothesis

Highly sensitive people (HSPs) make up about 15-20% of the population. HSPs process information more deeply and are more sensitive to stimuli such as sounds, light, and emotional events. They often can be identified already in early childhood as highly reactive babies and toddlers, who easily get overstimulated (e.g. by noise) and tend to be scared easily by things like clowns, the dark and often become shy or even suffer from social anxiety. HSPs are commonly found among artists and gifted people. Many HSP traits are also frequently found in autistic people (often minus the empathy).
Here are some common traits in HSPs:
  • High sensitivity to physical stimuli, including pain and temperature
  • High on trait neuroticism (overly cautious, worrying, overthinking, etc.)
  • High on trait “openness” (especially the aesthetic dimension)
  • Frequently socially  avoidant
  • Frequently have anxiety and phobias
  • Highly egalitarian
  • Highly empathetic and compassionate (often altruistic)
  • Non-competitive and non-violent (often even avoid violent media)
  • High in creativity
This bundle of traits makes sense when we consider it from an evolutionary point of view. HSPs have traits that typically associate them with the slow end of the life-history spectrum (later onset of puberty) and hunter-gatherer minds and geared towards ensuring long life-spans, i.e. long survival for the sake of parenting offspring that also takes longer to reach reproductive age. That is why they typically have a hyperreactive and usually bigger amygdala (the part of the brain that regulations emotions like fear and aggression).
 
HSPs are typically shy and cautious during their childhood during which they have the chance to learn about dangers and become insensitive towards innate fears. They are highly open to learning (openness traits) but in a cautious way. They are on the opposite end of the fast life-history strategy which manifests itself also in openness to experiences, but in an impulsive and risk-taking way (e.g. practising risky sports). NB: People with a female personality profile with fast life-history are also often HSPs (typically creative artists, actors, etc.).
Even though HSPs are relatively rare, they probably make up more than 50% of mental disorder patients. They have a process for depression, neuroticism and mental disorders such as bipolar and PTSD. HSPs are also physically sick more often, most likely due to increased stress levels (cortisol) that take their toll on the immune system. Like many other HSP children, I used to be a sickly child. HSPs are susceptible not only to colds and the flu but also to more uncommon reactions to stress, like hives, rashes and headache as well as allergies and asthma. 
Parenting HSP children is anything but easy they are quick to protest and cry (e.g. when having to go to school, the dentist, etc.). Such kids are often told not to be so sensitive, but they can’t really avoid it and the more they are criticized the more they withdraw. There are few things HSPs fear more than being criticized as their self-worth depends more on other peoples’ opinions than in non-HSP people. HSP teenagers typically tend to think of themselves as losers, in particular when male, as being highly sensitive and non-competitive creates that impression on their peers. Suicidal ideation is high in HSP teens when they aren't integrated, as is typically the case with gifted and ASD children.

One highly interesting fact about ASD people is that they are born with a bigger amygdala than neurotypicals, however, contrary to NTs the number of neurons decreases in ASD people (source).

The reason for this phenomenon has eluded science so far, but I speculate that it might be a protective mechanism, for the same reason that HSPs often become socially avoidant people: people who are born too "altruistic" for this world need to protect themselves. The amygdala is not only responsive to fear, but the oxytocin also drives altruistic behaviour via the amygdala. Abigail Marsh has established a connection between amygdala reactivity, recognition of fear in others and altruism. In a highly competitive world people who are born too altruistically have to be protective of themselves because it would be easy for others to abuse them. 


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