Toxic environments and human development

• TYPE OF PSYCHOLOGY: Biological bases of human behavior; Child and adolescent; Clinical; Counseling; Consulting; Developmental; Family; Health; Psychopathology; Social.

Exposure to toxic environments in childhood represents a potent risk factor for mental and physical illness across the lifespan. Advances in the research concerning early adversity suggest the biological embedding of early stress may be one explanation for this association. These findings offer promising avenues for prevention and intervention.

Introduction

Each year, a distressingly high number of children experience exposure to toxic environments. Terms such as "childhood adversity" or "early life stress" encompass a range of harmful experiences linked to increased lifelong risks for physical and mental illnesses, cognitive impairments, and diminished economic opportunities. Data from the US Child Protective Services indicated in 2023 that 7.4 out of every 1,000 American children experienced some form of maltreatment, including neglect or abuse. Additionally, many more children encounter adverse childhood experiences (ACEs), such as persistent poverty, domestic violence, parental loss, or other detrimental factors, which are known to impede healthy development. Climate-related disasters and displacement can also disrupt healthy development. These experiences can expose children to fear, loss, instability, and other stresses that affect mental health and well-being. Children may also face toxic experiences in digital settings. Online bullying, online sexual abuse, and repeated exposure to harmful content are linked to anxiety, self-harm, and suicidal thoughts or behaviors. In the 2023 Youth Risk Behavior Survey, the  Centers for Disease Control and Prevention (CDC) reported that about 76 percent of high school students had experienced at least one ACE, and over 18 percent had experienced four or more. A 2024 study found that over 80 percent of American adolescents experienced at least one ACE, and over 22 percent had experienced at least four ACEs.  Extensive social and behavioral science research has consistently demonstrated the profound effects of toxic stressors, highlighting their roles in anxiety, depression, conduct and personality disorders, substance abuse, obesity, hypertension, and cardiovascular disease.

Many theoretical models posit that toxic environments expose children to high levels of chronic stress and that this stress can shape psychological and biological development in ways that predispose them to later poor outcomes. Certainly, everyone experiences some stress in their life. For the most part, the sorts of stressors children encounter daily—such as entering a new social environment or negotiating with a sibling—may positively impact development. British child psychiatrist Michael Rutter described this as the ‘steeling effect.’ Exposure to mild and tolerable stress, especially in the presence of a supportive caregiver, can build coping ability that can be marshaled toward future environmental challenges in much the same way that an immunization inoculates against future infection.

However, toxic stress is different in that it is severe or frequent, can chronically activate the body's stress response systems, and may overwhelm the child's capacity to thrive. Toxic stressors are typically uncontrollable and experienced without the buffering effect of adult support, and introduce an immediate or chronic fear in the child for their physical or mental integrity. That is, children fear—often rightly—that they will be hurt or even killed. This fear can result from a direct threat or because they lack adult protection and are vulnerable to potential threats. These experiences, particularly during sensitive periods in early life, can derail normal brain development and can lead to heightened vulnerability to mental and physical illness.

Associations Between Toxic Environments and Developmental Outcomes

Childhood adversities are one of the most consistently documented and robust risk factors for psychiatric disorders across the lifespan. Epidemiological studies indicate that these experiences are associated with a large share of adult mental disorders. Toxic experiences rarely occur in isolation. For example, a child who experiences neglect is also more likely to live in poverty, be exposed to community violence, or suffer physical or sexual abuse. Research indicates that this accumulation of risk factors substantially increases the likelihood of later negative outcomes in a ‘dose-response’ relationship (i.e., the more risk factors a child experiences, the greater the likelihood of developing negative outcomes, including depression, substance use, suicidality, and chronic physical illness). Certain childhood adversities appear more strongly linked to adverse repercussions in adulthood; abuse or exposure to violence shows particularly strong links to later mental health disorders compared to adversities like parental separation or household instability.

Toxic stress has been associated with poor physical outcomes later in life as well. Among these are higher risks of substance abuse, heart disease, heart attack, stroke, asthma, liver disease, obesity, diabetes, sexual risk behaviors and sexually transmitted infections, smoking, and suicidality. People exposed to toxic stress as children are also more likely to become the target of domestic violence, have unplanned pregnancies or become teenage parents, be unemployed, or have less education.

The Biology of Toxic Stress

Theoretically, exposure to toxic environments, such as those characterized by adversity exposure in childhood, results in the body being exposed to toxic stress. Several coordinated biological systems govern the body's response to stress. First, the autonomic nervous system (so named because it is activated automatically and involuntarily) provides a rapid response by activating its sympathetic and parasympathetic branches. The sympathetic nervous system, providing what the physiologist Walter Cannon coined the ‘fight or flight’ response, releases epinephrine and norepinephrine from the adrenal glands to accelerate heart rate, divert blood flow toward skeletal muscles, and increase the production of glucose. The parasympathetic system, sometimes known as the ‘rest and digest’ system, in contrast, helps put the brake on sympathetic reactivity by slowing the heart and promoting growth. These two systems generally work in opposition to quickly allow individuals to prepare for and recover from threats in their environment.

A second biological system is known as the hypothalamic-pituitary-adrenal (HPA) axis, and provides a slower, more extended stress response. The HPA axis triggers a chain of chemical events that culminates in cortisol being released by the adrenal cortex above the kidneys. Cortisol, among other functions, mobilizes the body's stored glucose and allows it to deal with the energy demands required during stressful or threatening events. Numerous studies of human and non-human animals suggest that excessive exposure to stress can disrupt the functioning of the HPA axis, leading to a “wear and tear” effect on the body. Researchers also study epigenetic changes, including changes in DNA methylation, as one way early adversity may become biologically embedded and affect later health.

Studies using magnetic resonance imaging (MRI) have shown that chronic stress can negatively impact the developing brain, particularly in neural networks involved in emotion and learning. Two structures located deep in the brain, the amygdala and the hippocampus, are involved in ‘fear conditioning’ or the ability to predict threats based on past experiences. Experiments with non-human animals suggest that elevated levels of stress hormones, such as cortisol, can induce functional and structural changes in these regions. Children with experiences of toxic stress may more easily forge new fear memories or find it harder to unlearn old ones. These processes can trigger anxious behaviors that can result in internalizing disorders.

Sensitive Periods for Exposure to Toxic Environments

Over the last decades, neuroscientists have illuminated how the brain is shaped by experience and, in doing so, helped explain why toxic environments are so detrimental to the developing child. Studies using human and non-human animals have shown how the brain can be remolded or modified through experience: a phenomenon known as "neuroplasticity." Experience alters neural circuits according to the needs of the individual, and this allows us to learn and adapt to our social and physical environments.

However, the degree of plasticity in the brain is not the same across development. Brain plasticity continues through old age but is typically greatest early in life, when the brain is undergoing rapid growth. Thus, early environments have an exceptionally strong impact on the architecture of the brain. This period of heightened flexibility is known as a sensitive period.” Once a sensitive period is over and a neural circuit is fully formed, it becomes much harder to change in later life. This is why it is much easier to pick up a foreign language or become proficient at a sport if it is learned in early childhood.

The brain's sensitivity to environmental inputs in early childhood cuts both ways. On the one hand, children exposed to positive and nurturing environments are likely to reap the benefits of these experiences, providing them with a strong foundation for later emotional health. On the other hand, children exposed to deleterious or toxic environments may be rendered especially vulnerable as their brain circuits adapt to the kinds of adverse contexts in which they were raised. For example, children experiencing the threat of abuse in early life may develop a heightened sensitivity to the threat that persists even when they grow up and leave the environment in which they were maltreated.

An illustrative example of early adversity and recovery can be found in the Bucharest Early Intervention Project (BEIP), a landmark randomized controlled trial involving Romanian orphans raised in institutions. In this study, half of the children were placed in high-quality foster care, while the others remained in institutional settings. Over two decades, BEIP provided some of the most substantial evidence on how early deprivation impacts cognitive, emotional, and brain development ever collected. The findings showed that exposure to early institutionalization resulted in lower IQ, increased anxiety and depression, and difficulties with language and social functioning. However, children randomly assigned to foster care showed significant recovery in all these areas; recovery was greatest among children who left the institution when they were younger (e.g., twenty-four vs. thirty-six months). This supports the notion that the first years of life represent a particular sensitivity for later cognitive development.

Longitudinal research later expanded the BEIP’s findings through neurobiological research using imaging technology. The findings revealed that institutionalized children displayed widespread disruptions in white matter organization across the brain, but those placed in foster care (especially before twenty-four months) demonstrated more normative white matter development by adolescence. These findings supported the existence of “sensitive periods” of development in early childhood. Timely, enriched caregiving can partially reverse even profound early deprivation; delays in intervention may result in more developmental deficits.

Implications for Practice and Policy

The prevention of toxic childhood environments has proven to be a difficult problem and will likely require significant public investment in multifaceted intervention strategies working with families and communities. Nevertheless, converging evidence from psychology, neuroscience, and genetics has offered promising new ideas for the sorts of interventions that may be most fruitful for ameliorating the effects of disadvantageous environments in early childhood.

One such insight is that improving parents' ability to support their children is likely to be an important lever for improving child outcomes. Decades of research with human and non-human animals show that nurturing and supportive relationships in early life are essential to healthy development. In particular, primary caregivers have a particularly important role in buffering children from the toxic environments they may experience. Interactions with caregivers that rely on back-and-forth interactions with adults—what the National Scientific Council on the Developing Child called ‘serve and return’—are essential in teaching children to overcome stressful experiences. However, without these kinds of supportive relationships, children may be more vulnerable to the effects of toxic environments. Research has shown, for example, that children are more likely to experience elevated stress hormone activation if they have insecure attachment relationships or have a depressed parent who is unable to be responsive to their needs. Accordingly, interventions that focus on improving parent capacity (e.g., via home visitation programs or greater access to mental health services) may allow parents to be better able to buffer their children from the effects of toxic stressors.

A further insight is that knowledge gleaned from scientific research may help us improve interventions' developmental timing. As discussed above, the principles of neuroplasticity suggest that children are more likely to reap the benefits of intervention if it occurs early in life, when their brains are most responsive to environmental input. Enriching and stimulating early experiences lay the foundations for later learning, and so investments at this time will result in the best long-term outcomes. Indeed, economist James Heckman has found that social programs yield the greatest return on investment when implemented early in life, particularly before the start of formal schooling.

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