Do you ever wonder why people react differently to the same adverse rearing environments? While some children are permanently scarred, there are others who survive with no issues.
Are vulnerable children simply born with disadvantages?
It turns out that children with greater susceptibility have more advantages in other ways.
What is Differential Susceptibility
Differential susceptibility to environmental influence summarizes the observations that some individuals are disproportionately more susceptible to both negative and positive environmental conditions. It explains why people react differently to apparently the same environment1.
Children react to environmental effects, such as parenting style, differently2.
Recent studies have found that some children are affected more than others by family environments right from the start. They are more vulnerable to both positive and negative environments. This differential effect is summarized in the differential susceptibility hypothesis.
Differential susceptibility hypothesis
According to the differential susceptibility hypothesis, individuals with heightened susceptibility are more plastic or malleable. They are more sensitive to both beneficial and adverse experiences, especially in early childhood3. This enhanced sensitivity can result in enduring changes in youth development4.
This vulnerability is rooted in the biology of the nervous system. But besides neurobiology, adverse conditions during the early years (e.g. early child care) also play an important role5.
Diathesis Stress Model vs Differential Susceptibility Hypothesis
The differential-susceptibility hypothesis differs from the diathesis-stress model in how susceptible individuals are not just more vulnerable to negative environmental factors but to both positive and negative ones.
Exposure to childhood adversity, such as child maltreatment, unsupportive parenting, or negative life events, places children at elevated risk for developing antisocial behavior and mental disorders6. But not all children would develop negative child outcomes under such influences, and this is addressed by the Diathesis Stress Model.
Diathesis stress model (also known as the dual risk model) states that children with difficult temperaments or those who carry certain risk alleles (e.g. the s allele) are more likely to have maladaptive outcomes when subjected to certain stressors. They are predisposed or more vulnerable to certain health problems or psychological disorders than others.
The model, first proposed in the 1970s, serves to explain why some children are vulnerable to the negative effects of risky environments while others remain unaffected7.
Psychologists have found that this model is largely incomplete. It doesn’t lend itself to a convincing explanation as to why this is a desirable human trait.
The differential susceptibility model addresses exactly this shortcoming.
It states that some individuals are more susceptible to both positive and negative conditions, in a for-better-and-for-worse manner. Hence, susceptible characteristics are not liabilities, but plasticity traits8.
Given supportive and social environmental exposures, malleable children can reap the most benefit.
The Origin of Developmental Theories
Both supportive and adverse childhood environments have been part of human experience throughout our history. From an evolutionary perspective, differential susceptibility allows the human species to adapt to both kinds of contexts9.
Differential-Susceptibility Theory (DST)
Two different hypotheses have emerged to interpret the existence of differential susceptibility. One possible explanation comes from the differential-susceptibility theory.
DST, proposed by Jay Belsky at the University of California, Davis, suggests that developmental plasticity is based on evolutionary biology and natural selection.
Evolutionarily, it is advantageous for children to vary their sensitivity to the main effect of the environment because it is unclear what adaptation could maximize children’s chances of survival in different environments.
This property becomes a bet-hedging strategy in human evolution.
The less susceptible child will be less affected by environmental adversity while the more malleable offspring will benefit more from positive environments.
This genetic susceptibility allows the genes proven adaptable to the specific environment to pass on. As a result, humans, unlike other animals, can adapt to drastically different climates all across the world10,11.
Biological Sensitivity to Context Thesis (BSCT)
An alternative explanation is the biological sensitivity to context thesis. BSCT, proposed by Boyce from the University of California, Berkeley, and Ellis from the University of Arizona, suggests that natural selection allows a child’s stress reactivity to develop adaptive outcomes according to their rearing environment12,13.
In stressful environments, children’s chances of survival and eventual reproduction are enhanced if they develop heightened sensitivity and vigilance to threats. The adaptation to adversity “makes the best of a bad situation”14.
In supportive and enriched environments, reactive children adapt optimally in growth, status, fertility, and offspring quality.
Under the BSCT regime, differential susceptibility is the result of the stress-response system adapting to the environment in early life. Accordingly, the susceptibility factor is more of a developmentally-regulating strategy than a vulnerability or risk factor, even though those strategies may be detrimental to the long-term welfare of an individual or society as a whole under normal circumstances.
While DST addresses the role of nature in natural selection, BSCT addresses the role of nurture. Although these two theories are unique, they do not exclude each other. In fact, they converge on the fact that susceptibility to rearing influence is a desirable evolutionary, neurodevelopmental phenomenon in human development.
Differential Susceptibility Markers
The differential susceptibility hypothesis has shed immense light on why children react differently to the same environment or parenting style.
Researchers have identified several potential differential susceptibility factors, called plasticity markers. These markers predict the differential susceptibility nature when interacting with the environment15.
There are three types of plasticity markers:
- phenotypic (genetic factors you can “see”)
- endophenotypic (genetic factors “inside”)
- gene environment interaction
Here are some examples of these markers and the effects of their Parenting x Environment interaction.
Difficult temperament, defined as low inhibitory control, high dispositional frustration, high activity level, and low soothability, is a phenotypic factor.
In one recent study, researchers tracked the levels of externalizing behavior problems in a group of 16 to 19-month-old boys with difficult temperaments. After 6 months, the boys who had mothers with high parental sensitivity and infrequent use of negative control had the least increase in problem behavior. Those with highly insensitive mothers who relied heavily on negative control had the largest increase in behavioral issues. This effect was only found in the boys with difficult temperaments16.
Every parent dreams of having a baby with an easy temperament. These babies sleep better, eat better, throw tantrums less, and listen more. While having a difficult temperament predicts worse outcomes if the child is exposed to bad parenting17, it also predicts more positive outcomes if the child is raised with good parenting practice18.
Respiratory Sinus Arrhythmia & Cortisol Level
Respiratory sinus arrhythmia (RSA) & Cortisol reactivity are both endophenotypic factors (factor “inside”) of differential susceptibility.
RSA is related to an individual’s capacity to regulate stress while cortisol is linked to impulsivity, poor self-control, peer rejection, and aggression. These two markers are a gauge of stress reactivity, which in turn measures biological sensitivity to the environment.
In a study of 338 children the age 5 to 6, researchers have found that children with high biological sensitivity, as measured by RSA and cortisol levels, showed the highest levels of conduct problems in a negative family environment, but the highest levels of competence in positive family environment19.
5-HTTLPR is a genotype factor in the gene-environment interaction effects (denoted as GxE interactions). It is a polymorphism in the serotonin transporter gene area.
Neuroscientists have found that positive emotion was protective against psychiatric disorders. Children who have high levels of positive emotion are more resilient and have better self-regulation. Those who have low levels have a more negative affect. They are linked to more depression and other psychological disorders.
Neurodevelopment study also finds that young people who carry two functional copies of the S’S’ 5-HTTLPR allele are more genetically susceptible to the quality of parenting. They have significantly more positive emotions when reared in supportive parenting environments and significantly lower levels of positive emotions in unsupportive environments.
But those with at least one copy of the L 5-HTTLPR allele showed relatively consistent levels of positive emotion across both the supportive and unsupportive parenting environments. These findings are consistent with the differential susceptibility hypothesis20.
Differential Susceptibility and Parenting
Whether, how, and to what extent parenting affects a child’s development has been a longstanding interest to developmental psychologists, child psychology researchers, parents, and educators. The discovery of neurobiologically susceptibility has provided a much-needed additional perspective on this “does parenting matter” question.
A shared childhood environment can provide similar experiences, but it can affect different children to different degrees, including some not at all due to individual differences in genetic sensitivity.
Therefore, the effect of parenting on children’s development is neither as straightforward as earlier researchers suggested nor as insubstantial as behavior-genetics researchers suggest21.
As parents, while we cannot change our biological makeup, we can make the rearing environments safe and supportive for even the most sensitive children to thrive.
Furthermore, this dispels the myth that bad parenting practices are tolerated because of “different children require different parenting styles.” Different children simply benefit from good parenting to a different extent.
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