Childhood adversity refers to potentially harmful experiences that occur before the age of 18, ranging from abuse and neglect to growing up in a household marked by instability. Nearly two thirds of U.S. adults (63.9%) report experiencing at least one such event, and about 17% report four or more. These experiences are formally known as Adverse Childhood Experiences, or ACEs, and they carry measurable consequences for both mental and physical health that can persist well into adulthood.
The 10 Original Categories of ACEs
The framework for understanding childhood adversity comes largely from a landmark study conducted by the CDC and Kaiser Permanente, which identified 10 specific types of adverse experiences grouped into three broad categories: abuse, neglect, and household dysfunction.
Abuse includes three types:
- Emotional abuse: being regularly insulted, put down, sworn at, or made to feel afraid of being physically hurt by a parent or adult in the home.
- Physical abuse: being pushed, grabbed, slapped, hit hard enough to leave marks, or having something thrown at you by a parent or adult in the home.
- Sexual abuse: being touched or fondled in a sexual way, or subjected to attempted or completed sexual acts, by an adult or someone at least five years older.
Neglect includes two types:
- Emotional neglect: rarely or never feeling loved, important, or supported by your family, or feeling that family members didn’t look out for one another.
- Physical neglect: not having enough to eat, not having someone to take you to the doctor, wearing dirty clothes, or having parents too impaired by substances to provide basic care.
Household dysfunction includes five types:
- Domestic violence: witnessing a mother or stepmother being physically harmed or threatened by a partner.
- Substance abuse in the household: living with a problem drinker, alcoholic, or someone who used street drugs.
- Mental illness in the household: living with someone who was depressed, mentally ill, or who attempted suicide.
- Parental separation or divorce.
- Incarceration of a household member.
Each type counts as one point on a person’s ACE score, which ranges from 0 to 10. The score isn’t a diagnosis. It’s a simple way to quantify cumulative exposure to adversity during childhood.
Beyond the Original 10: Expanded Definitions
Researchers have increasingly recognized that the original 10 categories don’t capture every form of childhood hardship. Expanded ACEs now include experiencing poverty, bullying, community violence, foster care involvement, and discrimination. Some of these expanded categories, like experiencing racism or the deportation of a family member, disproportionately affect children from racially and ethnically minoritized groups. These stressors are often rooted in structural inequalities that perpetuate disadvantage across generations, and they carry similar biological and psychological consequences as the original 10.
How Many People Are Affected
Childhood adversity is remarkably common. CDC data from 2011 to 2020, drawn from nearly 265,000 U.S. adults, found that 23.1% reported one ACE, 23.5% reported two to three, and 17.3% reported four or more. Only about 36% of adults reported no ACEs at all. The sheer scale of these numbers means that childhood adversity isn’t a niche issue affecting a small population. It’s a defining experience for a majority of American adults.
What Happens Inside the Body
The human stress response is designed to be temporary. When you encounter a threat, your body releases a surge of stress hormones, including cortisol, that sharpen your focus and prepare you to react. Once the threat passes, the system resets. In children experiencing ongoing adversity, this reset never fully happens. The stress response stays activated for weeks, months, or years, a state researchers call toxic stress.
Under normal conditions, cortisol follows a predictable daily rhythm: levels rise just before waking, spike shortly after you get up, then gradually decline through the day. In people who’ve been chronically stressed from a young age, this pattern gets disrupted. Morning cortisol levels are often lower than expected, likely because the body downregulates its own stress system to avoid constant overexposure to stress hormones. The result is a new biological “set point” where the stress system no longer functions the way it should.
This process, sometimes called allostatic load, means the body is essentially stuck in a state of low-grade biological wear and tear. The changes are detectable surprisingly early. Adolescents with histories of child abuse or exposure to violence already show dysregulated cortisol responses, long before they reach adulthood.
Effects on Brain Development
A child’s brain is still actively building its architecture, which makes it especially vulnerable to prolonged stress. Brain imaging studies have found that childhood adversity is associated with smaller volumes in several key regions: the prefrontal cortex (involved in decision-making and impulse control), the hippocampus (critical for memory and learning), and the amygdala (the brain’s threat-detection center).
Research on adults with depression and a history of childhood maltreatment has found particularly strong effects in the anterior hippocampus, the front portion of the memory center, and in specific clusters of cells within the right amygdala. These are the areas most involved in processing emotions and forming emotional memories. The pattern suggests that early adversity doesn’t just create psychological wounds. It physically reshapes the brain regions responsible for handling stress and regulating emotions, which helps explain why the effects can feel so persistent.
Long-Term Health Consequences
The link between childhood adversity and adult health problems is one of the most consistent findings in public health research. People with four or more ACEs are nearly three times more likely to develop a chronic disease compared to those with fewer than four. The conditions linked to high ACE scores include heart disease, stroke, diabetes, chronic respiratory disease, liver disease, obesity, and depression.
The relationship follows a dose-response pattern, meaning the more ACEs a person has, the greater the risk. This isn’t just about mental health, though depression and anxiety are among the most common outcomes. The sustained biological stress described above damages the cardiovascular system, disrupts metabolic function, and weakens the immune response over time. A person with a high ACE score who develops heart disease at 50 may be living with the downstream consequences of stress that started at age 5.
Epigenetic Changes
One of the more striking areas of research involves epigenetics, the study of how life experiences can alter gene activity without changing the DNA sequence itself. Childhood adversity appears to change chemical tags on DNA (called methylation patterns) that control when and how certain genes are turned on or off. Early research has found suggestive links between physical or sexual abuse during middle childhood (around ages 6 to 7) and altered methylation in genes that regulate sensitive periods of brain development. Financial hardship during the same window showed similar patterns in genes involved in how those sensitive periods are expressed.
This research is still in its early stages, and no individual finding has cleared the highest statistical bar for certainty. But the overall direction is consistent: adversity during childhood may leave a molecular imprint that shapes how the body and brain respond to the world for years afterward.
Protective Factors That Buffer the Impact
A high ACE score is not destiny. Research on Positive Childhood Experiences, or PCEs, has identified specific factors that help counteract the effects of adversity. These fall into three broad groups: perceived relational and internal security (having at least one safe, reliable caregiver and beliefs that provide comfort), positive and predictable quality of life (regular meals, a consistent bedtime, stable routines), and interpersonal support (a caring teacher, a trusted coach, a close friend).
Even when stress is severe, a child who has a parent or caregiver capable of helping them regulate their emotional response can build resilience that changes the trajectory. Parental warmth, father involvement, positive peer relationships, school connectedness, and a sense of neighborhood belonging all reduce the risk for later depression and anxiety. The key insight from this research is that protective relationships don’t erase adversity, but they give a child’s developing brain and stress system the support needed to recover rather than remain stuck in a state of threat.