Personality Physics: the Discovery of Reactivity

A recent conversation with fellow parents sent me looking for what developmental research says about whether human beings are born with personality. Can we say that our neonate was born “a loner”, “affectionate”, or “opinionated”? Or do we lose credibility by straying beyond “calm”, “alert”, “irritable”, and “physically active”? Are personality traits coded in our genes? Or are they shaped by experience into the neural wiring in our brains? What I read surprised me. At the core of personality lies a nucleus, partly genetically determined, whose intensity has far reaching effects.

Philosophers have theorized about what makes us unique or similar since antiquity. They largely considered people as made – rather than developing – one way or another, which is why so many of us still today think about personality this way. It is only by the mid-twentieth century that developmental researchers started closely studying newborns and infants and following them through childhood, adolescence and adulthood. Their findings, synthesized in the Handbook of Child Psychology and Developmental Science (2015, 7th edition, Chapter 5 “Temperament and Personality” by Xinyin Chen and Louis Schmidt)  help us understand the dynamics that shape us, and pay attention to what really matters.

Born with temperament, not personality

In describing how newborns differ from each other, developmental researchers have distilled what is scientifically observable and roughly quantifiable down to a handful of dispositions: regularity of bodily rhythms, level of physical activity, reactivity to stressors, alertness, avoidance, attention focus, and such. They call these rudimentary traits “temperamental biases”, or “temperament.”

Temperament is biologically rooted. It is partly genetically inherited, partly conditioned by the environment in utero, and partly adjusted during the first experiences after birth. It is the initial nucleus on which a full personality gets built over months and years, through a cascade of billions of interactions with a dynamic environment – parents, caregivers, educators, siblings, peers, communities, cultures. Personality is a sophisticated set of skills, thoughts, beliefs, values, motives, goals, and coping styles which necessitate content that newborns do not yet have.

That laid the first part of the question to rest. The fascinating part was to learn about two aspects of temperament which have tremendous and lasting impact in shaping personality:

Reactivity and self-regulation

Some babies tend to be more anxious or overexcited and others more calm in response to novelty. It shows in their behaviors (eye contact, agitation, ability to calm down) and in their bodies (heart rate, breathing, activation of parts of the brain, cortisol levels, etc.) Researchers call this “reactivity.”

Reactivity indicates how sensitive an infant is to stressors and how strongly it will express its distress or excitement: ‘highly reactive’ babies are those who display high levels of physiological arousal, either hypervigilance or exuberance.

Although it can ensure short term survival in the wild – “the squeaky wheel gets the grease” – research shows that high reactivity is a risk factor for maladaptive development. Longitudinal studies have established that highly reactive infants measured at 4 and 24 months tend to be less sociable in childhood; display higher levels of social anxiety in adolescence; and have delayed transitions and more difficulty with major life roles (careers, marriage) in adulthood.

High reactivity is a risk factor, not a curse. We know the following about how it works:

First, genetics are at play. We know this because animals can be bred to produce highly fearful or aggressive strains, and because identical human twin studies have shown them to be more similar in fearfulness than fraternal twins. A couple of genes seem to be implicated: the dopamine 04 receptor gene DRD4 which affects novelty seeking, and the serotonin transporter gene 5-HTT which is linked to anxiety and withdrawal.

Second, pregnancy influences gene expression. Numerous studies demonstrate that “maternal prenatal emotional stress is significantly associated with infant affective reactivity to novelty.”  According to Brazelton, who developed the Neonatal Behavioral Assessment Scale, a newborn’s behavior is “phenotypic at birth, not genotypic.

Third, reactivity is affected by early experience and parenting styles. “Oversolicitousness and overprotection,” for example, “are developmental origins of children’s reactive and shy-inhibited behavior.” 

Fourth, reactivity is modulated by another critical early-developing trait, self-regulation, ie the ability to focus attention, cope with negative experiences, and adjust behavior. Self-regulation develops (it can also over-develop or under-develop) through the attachment relationship with caregivers: “Parents may affect children’s regulatory abilities in several ways, including serving as models in managing stress and negative emotions and providing warmth and support that are beneficial to the development of secure relationships. Parental insensitivity and unsupportive reactions induce aroused and dysregulated emotions in children.”  Repetitive routines and a sense of control helps infants strengthen progressively their ability to self-regulate without becoming over-sensitized by overwhelming levels of stress. Touch is another important way in which infants receive support: “Early experience of body contact or proximal parenting is conducive to the development of self-regulation.”

In summary, reactivity and self-regulation are the magic duo that has the most enduring and far reaching impact in our personality development. Together, if out of whack, they lead to dysfunction. “Researchers have consistently found that, in Western societies, (1) reactivity to stressful situations and behavioral overcontrol predict internalizing problems such as anxiety and depression, while (2) reactivity and behavioral undercontrol predict externalizing problems such as aggression and antisocial behaviors.”

Setting the dial has snowball effects

The belief that children come with their personalities genetically set distorts our understanding of the impact we have on them. Our biggest impact, it turns out, is at the very beginning, when it is the hardest and when it seems the least impactful.

During the first months of infants’ lives, caring for them is a grind: many of us are recovering from a pregnancy, sleepless, socially isolated, and have a hard time understanding their needs; infants are unpredictable and not yet interactive. Caring for them can also be unrewarding: most adults find it difficult to abnegate their own needs and interests for a prolonged period of time during which they dedicate themselves almost exclusively to holding, cooing, and cleaning an infant in endlessly repetitive routines.

However, understanding how critical it is to set reactivity and self-regulation at balanced levels hopefully encourages us in this seemingly thankless and amorphous effort. Being more objectively observant and affectionately present in the early months, we become vigilant to risks for maladaptive development, quicker to intervene, and more effective in preventing serious issues in later life. Curiosity, sociability, compliance, determination, empathy… and other qualities like these flourish more readily around a well-adjusted nucleus.



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